When I started playing on the PC version of the game, the first thing I checked was if there was any missable in the game. Most people at the time were pointing out only the Obsidian Garb... In fact there is more than that, and the main reason people didn't know them at the time was because the switch version didn't have all those achievements to unlock, so you can imagine that all that matters at the time was to get every piece of gear and that's all.

Because I like the game so much and wanted to unlock every achievement, of which i'm very proud of having done, I did face the problem of having to reset my progress more than once because I've missed something. I think I've thrown out the window more than 40 hours of gameplay, because i didn't have the information at the time and I learned the hard way. My friend got in an even worst situation having spent more than a hundred of hours and in the end trying to complete his item's list, finding that he was missing 3 piece of knowledge and probably at least 1 valuable he couldn't acquire any longer without restarting from scratch, these were the only thing he was missing when comparing to my item's list and I got the achievement while he had to restart his game.

Fortunately there isn't too many so the list isn't too big, but if you don't want to throw many hours of gameplay for missing something then, you may want to check the following.

This is my newest guide which includes a spreadsheet to assist you during your playthrough. Every missable and limited items have been highlighted in the spreadsheet so this gives you a powerfull tool you might want to use: https://steamoss.com/sharedfiles/filedetails/?id=2309451625

• When you reach Cyrus chapter 3, you'll be dump into a pit in "Yvon's Birthplace". Don't miss out on the chess down the pit. This is the only chest you can miss.

I wasn't planning to share this list at first, but some people were pointing out to me errors in other listing and some friends told me they prefer my document, so if you like it then you have the option of using it, if you want to work with the file, you can "File -> Make Copy" for yourself to have edition rights and use the checkbox as you like: https://docs.google.com/spreadsheets/d/1bLovPjTQpj12l6MW3axDQRG0wuRpN_a_1E086Kw9WoA/edit?usp=sharing

• In S'warkii, Lord Ciaran's Herald. This NPC is available only during H'aanit chapter 1. So just make sure you have either Cyrus or Alfyn before getting H'aanit. This one has been tested as not required for the achievement, so if you missed it, it doesn't matter too much.
  
• In Clearbrook, Zeph has a hidden item, he normaly stand right outside his house. When you have finished Alfyn and Cyrus chapters 4, Zeph moves inside his house ready to start his sidequest with Mercedes and he no longer have his hidden item available.
  
• Also in Clearbrook, Meryl can be found at the graveyard. After completing her first sidequest "Meryl, Lost then Found (1)" she leaves Clearbrook and you can no longer acquire her hidden item afterward even though she returns to Clearbrook at the end of all her sidequest.
  
• In Wellspring, Bale can be found inside the barracks behind the table, his hidden item is available after you initiate Olberic's chapter 3 and Bale disapear after you finished Olberic's chapter 4. Even though he returns after you finish both Ophilia's and Olberic's chapter 4 (to give you one of the best reward in the game through his sidequest), he doesn't have the hidden item anymore when you Scrutinize/Inquire him.
  
• In Duskbarrow, a NPC named "Crest-bearing Swindler" can be Scrutinized/Inquire for a hidden item and he is also part of a side-quest "The Price of Vengeance" where you have to Challenge/Provoke him with either Olberic or H'aanit, if you win the Challenge/Provoke he runs away and you can't find him anywhere else after, so be sure you get the hidden items before fighting him. Thanks to Jabberwork for the info.
  
• In Trail to the Whitewood (Frostlands Tier2), a NPC named Traveler appear once you started the quest "Arianna Again (I)" and he's holding a hidden item (Inspiriting Plum (M)). That quest can be started by talking to Arianna after you have finished Primrose and H'annit chapters 4. When you complete the 2nd part of that quest "Arianna Again (II)" by bringing Oren to Natalia in Stonegard the NPC Traveler doesn't have the info about the hidden item anymore so be sure you get it between those 2 quests. Thanks to Doid for this one. (I got a report from Railgun and other sources saying they earn the achievement before inquiring this guy, so it looks like this one doesn't count toward the achievement)

• Obsidian Garb: The only place you can get this is against Primerose 3rd boss fight, "Albus, the Right-hand Man". You can steal the Obsidian Officer to get it, and the 2 Obsidian Officer have a 30% chance to drop it after the fight. If you want the maximum number available for a perfect savefile, dont forget to equip the Snatch passive (3rd Thief passive), this will allow you to potentially get 6 of them. I got a report from Hajime Saito claiming he got the achievement while missing the Obsidian Garb and this has been confirmed by someone else.
  
• Original Tome (Valuables): After killing Lucia (Cyrus chapter 4), you'll have to scrutinize 3 pieces of Knowledge:
  • Trial of the Twelve, Volume VII
    
  • Forbidden Gold
    
  • From the Far Reaches of Hell
    It is important you get "From the Far Reaches of Hell" last otherwise you won't have the Original Tome in your Valuables. So scrutinize first the books in the old library and then checkout the book at Lucia's body. As soon as you scrutinize your 3rd Knowledge, if you did that in the right order, you should have the Original Tome in your inventory.
• Joshua's Weak Points (Knowledge)
  During Olberic Chapter 2, be sure you Scrutinize/Inquire Joshua before you fight him.
  
• Archibold's Weak Points (Knowledge)
  During Olberic Chapter 2, be sure you Scrutinize/Inquire Archibold before you fight him.
  
• Gustav's Weak Points (Knowledge)
  During Olberic Chapter 2, be sure you Scrutinize/Inquire Gustav before you fight him.
  
• Tidings of Elderly Friends (Knowledge)
  During Alfyn Chapter 3, you'll have to Inquire a Townsperson for "Tidings of Elderly Friends". It's kinda hard to miss, you should see him right away upon exiting the tavern, but you can totally skip him if you go Inquire Demure Grandma right away. Thanks to Weichichi for this one.

I got a report from VictorWonder and multiple other sources saying they earned this achievement while missing those knowledge about the 3 weakpoints from Joshua, Archibold and Gustav.
Also thanks to Soulcast for letting me know "Tidings of Elderly Knowledge" is not required for the achievement.
While I don't know for sure if other Knowledges are required or not to complete the achievement, I know for sure that Valuables are so be sure you don't miss the Original Tome, this one has been specifically tested as required for the achievement.

• Meep: found in the main areas of the flatlands, tier 1.
  
• Cactus Roller: found in the main areas of the sunlands, tier 1.
  
• Sea Birdian 4: found in the main areas of the Coastland, tier 2.
  
• Buccaneer 1: found in the main areas of the Coastland, tier 2.

Meep and Cactus Roller can only be found when you have 3 or less characters in your party. When you get your 4th character the Tier 1 zone will increase to Danger Level 11 and they don't appear anymore at that level.
I got a report from Soulcast saying he got the achievement while missing Meep and Cactus Roller, again if you don't want to take any chances be sure to get them however it might still be possible to obtain your achievement without them. However Sea Birdian 4 and Buccaneer 1 you'll need them because from everyone i've heard missing those 2, they were not able to get the achievement.
For the Sea Birdian 4 and Buccaneer 1 you'll have to get them before the zone increase to level 34. Doing chapter 2 and beyond is what will make your Tier 2 areas increase in level, so this would be a good thing to get those 2 enemies before engaging in any Chapter 2 story.

The following are just entries in the Quest Journal, they aren't related to any achievement however if you want a complete Journal, these are what you don't want to miss.

• Ophilia's, "True Belief": Just make sure to talk to Mattias before going out to the Sea side grotto during her chapter 3 quest.
  
• Therion, "Robbing a Robber" & "Slacker": During his chapter 4, just make sure you steal the Brigand Garb first and check the optional cutscene in the Lorn Cathedral before getting the Brigand Leader's Garb.

This section covers the Limited items in the game that you can miss. What I mean by "Limited" is you'll be limited in the total quantity of a specific item you can get and if you miss one of the following, then that's one less copy into your inventory.

If all you care is getting the achievements, then you can dismiss entirely this section because none of the items in this list will prevent you from getting any achievement because they can all be acquired through other means that isn't missable. However if you are a perfectionist and doesn't want to miss at any opportunity to increase your stack of limited quantity items, then check out the following.

You'll see that for many of them you'll have to steal against specific enemies that you can encounter only once during your playthrough. When that's the case be sure you have your Snatch passive enabled (3rd Thief passive) in order to double your spoil. Also the steal table is always the same as the drop table, so if you can steal a monster something, he can also drop it, the other way around is not always true, some boss can drop unique weapon and you won't be able to use steal on them. As for normal monsters, they don't always drop their stuff, many of them have 30% drop rate (unsure if there are different percentage for different monsters), bosses have always 100% drop rate but this doesn't apply to their adds which will have lower drop rate (usually 30%). If they don't drop their stuff, reset your game and try again until you get what you are looking for.

• Handaxe: This is H'aanit and Alfyn starting weapon but you can acquire more by stealing against the 2 "Brigand" during Olberic chapter 1 Boss. In total you can have up to 9 of them. 2 from starting weapon, 4 of them you can steal the Brigand, they can also drop 2 (reset until you get both of them) and you can Buy/Steal another one in Quarrycrest.
  
• Longsword & Heavy Coin Pouch: The Longsword is Therion and Olberic starting weapon, you can acquire more by stealing against Helgenish's Lackey during Primrose chapter 1 boss battle. After having killed his lackey, Helgenish summons additional ones after a few turns, so you can steal 99 of them if you want. This is your only chance to get more than the initial 2. Helgenish himself is holding Heavy Coin Pouch, be sure to steal him as well with Snatch passive.
  
• Tree Nut: During Tressa chapter 1, this is available only after the first little cut scene with Captain Leon where you need to buy the "Sleepweed" from one of the 3 "Merchant". You can miss the item if you chose to not "Hear the begining of the tale" from Tressa or if you buy first the Sleepweed you won't have any other chances to buy from the other "Merchant" who sells you the Tree Nut.
  
• Exotic Garb: During Olberic chapter 2, you'll have to challenge 2 NPC before being able to fight Victorino. One of them is named "Prideful Warrior" and he can drop the Exotic Garb. Restart your game until he drops the item.
  
• Ice Amulet: Buy/Steal the Ice Amulet from Derryl in SaintBridges before finishing Ophilia chapter 4, after that his inventory changes and the Ice Amulet is no longer available from him.
  
• Divine Blade: During Olberic chapter 3, you'll have to fight Erhardt. Be sure you have the Thief subclass equiped with Snatch passive. You should always have the drop but be sure you steal it for more copies.
  
• Silver-filled Pouch: During Therion chapter 3 you'll have to steal either an Attendant's Mask, Aristocrat's Mask or the Black Market Inventory. During this little sequence, you can steal a Silver-filled Pouch from Employee.
  
• Black Staff & Stimulating Bracelet: During Ophilia chapter 3, you can steal Mystery Man & Shady Figure to get theses 2 items. Again they will always drop the item but steal them to get more.
  
• Imperial Lance & Elemental Glaive: During Tressa chapter 3, you can steal/buy Captain Leon those 2 items. Do so before you end the chapter. Thanks to Doid for this one.
  
• Rune Knife: Steal Esmeralda during Tressa chapter 4 boss fight, she always drop it at the end.
  
• Red Apple: Steal Darius during Therion chapter 4 boss fight. You can only have 3 maximum of that item.
  
• Sharp Nut (L) & Sprightly Necklace: Steal Grave Robber in Moldering Ruin (Woodlands Tier 3) after you Challenge/Provoke him it completes the quest "Looting Grave Robber" and he go away never to be found again.
  
• Inferno Amulet: Buy/Steal from Wayfaring Girl in Sunshade during "Ria, Born to Roam quest (I)". After you Challenge/Provokee Ne'er-do-well it completes the quest and she moves to Wellspring under the name Ria (even though her real name happens to be Gloria, this can get confusing but you'll see that during her 3 sidequests). Once in Wellspring even though you got the item in Sunshade her inventory resets for no apparent reason but it allows you to buy/steal another Inferno Amulet.
  
• Tree Nut & Dubious Gold Ore: Buy/Steal from Kaia during her quest: "Kaia Mother of Dragons (2 or 3)", once you completed her 3rd quest she goes away and you can't find her anymore. She will be in the cliftlands, so during her 2nd quest she is at Quarrycrest, and on her 3rd quest she is at Orwell.
  
• Introduction to Adventuring & Old Coin: Buy/Steal from Le Mann. You can get those during any of his 3 quest, in Rippletide, Goldshore or Grandport but once you complete the 3rd quest he goes away and you can't find him anymore.
  
• Glass Marble: Buy/Steal Crest-bearing Swindler in Duskbarrow before attempting any challenge/provoke on him. This is also the same guy having a Hidden Item to scrutinize/inquire in the section above.
  
• Enchanted Sword: Challenge/Provoke Alphas. This NPC can be found in multiple places depending what quest you've completed. At first he starts in Bolderfall, then once you start Daughter of the Dark God (2), he moves to Saintsbridge. As soon as you complete that quest he will stay in Saintsbrige but he'll move near the Cathedral. When you start the Quest: Alphas and the Impresario, you'll see Alphas in West S'warkii Trail and this is your last chance to Challenge/Provoke him. Once you complete "Alphas and the Impresario" by defeating the tiger you won't be able to challenge/provoke him anymore. So if you want more Enchanted Sword this is what you have to consider. Also when Alphas is in Bolderfall he is only 5 star of strenght, while in other places he gets buffed with 8 stars, so it's up to you but he's weaker to fight in Bolderfall. Last thing is this is a 5% drop rate only, so don't expect it to drop often, so maxing your stack to 99 is definitely possible but that's kinda insane, this is one item I got a few of them (6 or 7) and then I hacked my savefile to have 99 (because I like that beautiful number but at the same time I can't waste a year just for that). So if you want more Enchanted Sword this is what you have to consider.

This guide has become larger and larger as new things have been found over the time. Now in its current state the information above can be hard to sort out. So this is why I'll provide to you a road map for your playthrough to follow to make it easier. The following holds the same as all the lists above but they are regrouped by Chapters so it's easier to follow if you are in the middle of your playthrough and you don't want to miss anything.

• Chapter 1
  • Limited Item / Eagle-Eyed (Hidden Item): When choosing your first character, anyone except Olberic, Primrose and H'aanit. As for Olberic and Primrose you want someone with the Steal ability to use against their chapter 1 boss. And as for H'aanit, you'll want to have either Cyrus or Alfyn in your party to get the hidden item from Lord Ciaran's Herald. Cyrus and Tressa are very good choice to start with, Cyrus will provide efficient way to reveal weaknesses and Tressa's talent to purchase stuff from NPC is exceptionally good if you're able to get the most of it (if you want to go fast check out my "Perfect Playthrough + Resource Guide" I added some video at the end of the guide to show you how to cheese the game).
    
  • Strategist (Enemy Weakness): Once your first chapter 1 complete, you can get 2 missable monsters in Tier1. Meep (Flatlands) and Cactus Roller (Sunlands). Get those 2 monsters weaknesses before acquiring your 4th character and you're not allowed to complete any chapter 2 before getting those 2. From reports I got, it was later found out not to be required for the achievement, however if you still want to reveal those weakness that's what you have to consider.
    
  • Limited item: When starting Tressa chapter 1, be sure you chose to "Hear the beginning of the tale". When you first meet Captain Leon you'll have to buy the Sleepweed item, but before you do that, be sure you buy the Tree Nut available on one of the 3 Merchant (left most).
    
  • Limited items: After you completed Tressa chapter 1 you can Buy/Steal Le Mann in Rippletide for his Introduction to Adventuring & Old Coin. If you already missed it, you have another chance to get it in Goldshore and Grandport.
    
  • Eagle-Eyed (Hidden Item): While doing H'aanit chapter 1 make sure you get the Hidden Item from Lord Ciaran's Herald, after that the NPC is no longer available.
    
  • Eagle-Eyed (Hidden Item): Get Zeph and Meryl hidden items in Clearbrook. Zeph shouldn't pose any problem because it's much later in the game you loose the ability to get his hidden item, but be cautious with Meryl, you need to get her hidden item before completing her sidequest.
    
  • Limited items: When ready to start with Olberic and Primrose chapter 1, make sure you have the ability to steal with Snatch passive to get the maximum number of Handaxe, Longsword and Heavy Coin Pouch from the bosses. Check the Limited item section if you need more info on this.
    
  • Limited items: After Primrose joins your party you can Buy/Steal Ria (Wayfaring Girl) in Sunshade her Inferno Amulet, be sure to do that before Challenging/Provoking Ne'er-do-well as this will make her move to her next destination (her Buy/Steal inventory gets reset in Wellspring).
  
  
• Chapter 2
  • Limited items: Buy/Steal a Dubious Gold Ore from Kaia in Quarrycrest. This requires you to complete Kaia Mother of Dragon (i) in Bolderfall. If you already missed it you'll have another chance at getting it in Orwell.
    
  • Strategist (Enemy Weakness): Get Sea Birdian 4 and Buccaneer 1 weakness in the Coastlands tier 2 before completing any chapter 2. Your last chance at getting those 2 is when you have 5 chapter 2 quest completed, when you complete your 6th it's too late.
    
  • Olberic
    
    • Limited items: When challenging Prideful Warrior, make sure he drops the Exotic Garb before continuing. If he don't reset to restart the fight.
      
    • Optional Knowledge: Inquire or scrutinize each bosses to get their weak points before fighting them in the arena.
  • Limited items: Buy/Steal the Ice Amulet from Derryl in Saintbridges, once you complete Ophilia chapter 4 his inventory changes and he no longer has this item available.
  
  
• Chapter 3
  • Ophilia:
    
    • Journal Entry: Talk to Mattias before heading to the Seaside Grotto for an optional cutscene. This should add "True Belief" into your journal.
      
    • Limited items: Steal the Mystery Man & Shady Figure for Black Staff and Stimulating Bracelet during your boss battle. Be sure to have Snatch equip.
  • Cyrus: Treasure Hunter (Chest): When you're in the pit be sure you grab the chest.
    
  • Tressa: Limited items: Steal/Buy Captain Leon the Imperial Lance & Elemental Glaive during Tressa's chapter 3.
    
  • Olberic:
    
    • Eagle-Eyed (Hidden Items): Inquire/Scrutinize Bale in Wellspring.
      
    • Limited items: Steal Erhardt the Divine Blade when facing him. Be sure you have the Thief subclass and Snatch support equip when facing him. You won't be able to reaveal all weakness with Thief job equipped however Bosses and Unique enemies don't count for the Strategist achievement so this won't lock you out of any achievement. Unfortunately you can't have both but personally I value the item more than the weakness.
  • Primrose: Collector (Aquired all items): Steal the Obsidian Garb against the Obsidian Officer during her boss battle. For maximum amount have Snatch support equipped and make sure they drop 2 at the end of battle, otherwise reset until they do.
    
  • Alfyn: (Optional Knowledge): Just be sure you don't skip Inquiring Townsperson during Alfyn chapter 3 to get "Tidings of Elderly Friends" (knowledge) before getting to Demure Grandma. It's kinda hard to miss but totally possible.
    
  • Therion: Limited items: Buy/Steal Employee the Silver-filled Pouch. You can get this only during the little sequence where you need to buy either a Mask or the Black Market Inventory.
  
  
• Chapter 4
  • Eagle-Eye (Hidden items) & Glass Marble (Limited items): Inquire/Scrutinize "Crest-bearing Swindler" in Duskbarrow for his hidden item & Buy/Steal the Glass Marble on him before you attempt any Challenge/Provoke that will make him run away.
    
  • Limited Items: Buy/Steal Grave Robber the Sharp Nut and Sprightly Necklace in Moldering Ruin (Woodlands Tier3) before any attempt at Challenge/Provoke, as this will make him run away.
    
  • Cyrus: Collector (Acquired all items): After killing Lucia be sure you scrutinize "From the Far Reaches of Hell" last to get the Original Tome (Valuables). Check out the Collector section above if you need more precision on this.
    
  • Tressa: Limited items: Steal Esmeralda the Rune Knife with the Snatch support equipped.
    
  • Therion:
    
    • Journal Entry: Steal Brigand's Garb first, before stealing the Leader's Garb and check an optional cut scene by entering Lorn Cathedral, this will add both "Robbing a Robber" & "Slacker" in your journal.
      
    • Limited items: Steal Darius the Red Apple with the Snatch support equipped.
  
  
• Post game
  • Eagle-Eyed (Hidden items): After you have completed Primrose and H'aanit story line go talk to Arianna in Stillsnow to start her sidequest. Then go Inquire/Scrutinize "Traveler" in Trail to the Whitewood to get his Hidden item. You need to get this hidden item before you complete the 2nd part of the subquest.
    
  • Limited items: Get as many Enchanted Sword as you like by Challenging/Provoking Alphas in either Bolderfall or Saintsbridge depending which side quest you've done. Just be sure the Side quest Alphas and the Impresario isn't completed. More explanation in the Limited item section above.

If you ever find a new missable in the game. Please let the community know. You can write a comment down here and I'll take the time to add the info and give you credit for your finding.

Thanks to everyone contributing to this effort.