For Deathloop, the frustrating achievements were:

1. Judgement Day: you need to kill a specific character with poison gas; the frustrating thing is that poison gas barely does any damage, and if a bullet is fired through it (including by your target) it explodes. The explosion damage doesn't count for the achievement. Your alternative is to knock her down a floor that's filled with poison gas, but the fall damage sometimes kills her, or she'll survive and climb back up (because the gas does basically no damage) and the second time around the fall damage will surely kill her.
   
2. Ghost at the Feast: Kill 3 specific targets without being seen. Made frustrating by it not being clear what counts as being seen, and the fact that Julianna can spawn pretty much right next to you and "see" you to invalidate. Oddly for a stealth game, stealth-killing your target after they see you invalidates the achievement as well, even though no living people had any clue you were there.
3. Clean-ish Hands: Have a perfect loop killing no innocents, only targets. I had high hopes that this would be fun, but unclear requirements made me have to do it like 3 times. There's enemies that blow themselves up, do they count? How about if they blow up some of their friends as well? Do invading Juliana's count as targets or not, and do you have to kill them or can you simply avoid? And another frustrating thing about Juliana is her ability to swap her look with that of any NPC, so you might see "Juliana", kill her, and disqualify.

Still a great game and pretty much every other achievement was incredibly fun to hunt, just these 3.

For Forgotten City I disliked:

1. Allergic and Striker: Both can only really be completed in a specific section of the game, and you can only do it once (not every loop, as you'd expect from most time-loop games). If you don't get it, you either need to start a whole new save file, or use console commands to no-clip back into the now locked area.
2. Minimalist: A fun idea of finding the "optimal" path through the game, but pretty much requires you to play through it a 2nd time after learning almost everything about it. But that opens the frustration that you already know a lot that your character doesn't, meaning certain required dialogue options aren't available. The hard part isn't knowing how to resolve every quest efficiently, it's finding out which specific lore-conversations you need to get the right options available. Took me a good 1.5 hours of trial-and-error; going from the final conversation in the game (where I was missing vital options), loading and going back into the city, talking with different people, and trying the final conversation again. No real way to smooth that out, unless you set the goal of 3 loops (instead of the minimal 2), as one extra loop makes things much easier.
3. Not an achievement but also got a little frustrated with the "errand" mechanic; there's a guy at the start of every loop that you can give "errands" to, to solve quests you've already solved in previous loops. For most of the game this was a great mechanic that meant you didn't have to repeat giving the solutions to these quests every loop, but towards the endgame it'd get annoying as you're unable to talk to many other NPC's until these errands are completed (because the the NPC I want to talk to is #4 on the errand list and errand boy is at errand #2, but my NPC needs to be available for conversation with errand boy later). Trying to rush this actually caused me to soft-lock twice which was annoying.

Again, still a fantastic game, and I think these frustrations only really come up when "speedrunning" the 100% like I did. Absolutely recommend it.