Destiny - Not good but "whatever": Boring repetitive corridor chunked around into locations that just froze your game for a while to load itself out of the blue. Very grindy game, but sometimes you want to play some grindy game, so ok.
But.. they made some 2nd (or something) DLC (called Iron Fist or whatever) and they locked some needed in-game currency or what behind that new DLC. Without that it was pointless to play, because you couldn't craft any usable gear. They just took the most important and already paid part of the game away from you until you payed again, because of milking and why not. It was a scam to me. Until today, I have no clue how they got away with it (legally too). They made it to Destiny 2 and got even bigger hype that quite annoyed me. Games-related media didn’t cover that, but those are just shared PR for companies and I never felt like they actually do games much, no shocker there. But.. I didn’t see players talking about it and it was weird. Fortunately PS was not mine and only reason why I didn’t care further. Bungie is a big no-no since.

Anything Nintendo. I got Switch from a friend because he was bored of it and I am trying to get rid of it too, from many reasons. Games-wise: I don't remember a single Nintendo character that didn't bother me visually or/and audio-wise. In my view, it's just all jumping simulators full of psychopathic and spoiled characters and that hype is all because of some memberberries phenomena. Those games are not such a fun and not that good.

Zelda (BotW) – Very boring and pointless game with bugs (monster respawn went berserker on me so I couldn't play as intended). Charming world ended up in the moment when any humanoid (except monsters) came along or I looked at Link. Everything alive in Nintendo games is annoying to my taste. “RPG” where you collect stuff that breaks after few slashes (you can't repair and inventory itself is quite small).
I never got that game itself and that hype too. Not saying it's bad, just not so good.

Smashbros - Annoying to play online and almost like it was designed to.. smash.. your already broken controllers. That interface/menus are the worst ones I seen, ever.

Mario Kart - Just sucks in my opinion, there is not much to say. That game is pointless to me. Utterly boring to play even with friends. I prefer FlatOut 2 or something actually funny to play. It's one of the most boring games I ever played and I never had any desire to put it on by my own.

It's true that I can’t stand Nintendo as a whole from many reasons, but that is the letdown I learned. I personally detest (hey, I can) any japa-anime-whatever style games (big swords and extremely annoying characters) with my heart so it didn’t help in this case. Still I was letdown, because of the unjustified hype that I still don’t get. I didn't expect it being that bad even gameplay-wise.

Doom 2016 - On Switch looks so bad and blurry so I never played it again after one playthrough (didn’t even care about collecting everything) and multiplayer had exactly 0 players all the time. Yet, premium price. And yeah, I found it boring and mediocore and somewhat static, plastic and basic. Well, and also who seriously plays shooters with a gamepad :D (joke.. maybe). UT2004 (well, on PC) much better even today. No really, why it was so hyped too? Nothing special.

South Park SoT - Al Gore. That is all I say. At that point that game was not even funny and I never played it again. And to deal with him was for me in my situation impossible and I spent a very long and a very annoying time with it. I just never started it again. Also, that game was a bit too much of homage to itself without actual game underneath. So it ended up being quite shallow.

NWN - What's the point of making RPG where you go alone because of it's XP system. It would be ok if your summoned monsters wouldn't cut your experiences down too. Mages don't do weapons. Who had that idea? What a letdown. But at least good platform for mods and DLCs not that bad. Still like it tho.

Dragon Age Origins - Except Shale, I unfortunately found all female characters (I met) extremely annoying, spoiled, cringy, childish and out of the place (not because of a gender - of course - but 'cuz how they were made as a whole - to be clear in these ridiculous times) and I think that biggest problem were voice actors and even some face animations didn't help, except Shale. Shale and Alistar had a very good voice actors and I liked their personalities. I really liked that game itself and some characters were fine, but Leliana was such a cringe and annoyance that solely forced me to uninstall that game even while “I didn’t want to”. And she is quite forced in that game and into your face. Even in DLCs.. that didn't quite follow it's own lore.. (and at least one is just about her) and you have to finish them to get some stuff and sets. I couldn't. I tried. I was about to kick her out and took her armor off (to sell) and she jumped into what I was doing and she started singing to me in the camp in underwear with the most weird camera angles and I had to suffer through that all anoying part because I didn't want to loose lore. I actually laughted my ass off with that amount of cringe in that moment. I tried to ignore her but it's impossible story/DLC wise. I can't stand her voice, her acting, her self-delusional tendencies. She is just absolutely terrible and annoying person in every single aspect. She is spoiled and immature and nowhere near of position of some hero killing anyone, yet that game is trying to put it like there is more value to it and it's all not working. Well, all of those characters tend to ignore what they do and seem to be a bit spoiled and like they don't live there at all. But she is not believable as a concept in that game's world. Morrigan was the same annoyance too, also with annoying voice acting and behavior.. but Leliana is for me seriously gamebreaking and the worst and pushed character in any game, ignoring all those Nintendo ones. The only one coming closer is that new mage in BG:EE, but I don’t play EE. :D I think they tried hard to make Leliana "the babe" and did the opposite.

Beamdog - Everything they done and everything they are. So I will not list all of those games itself. It's all of them.

Wasteland 2 - Not sure if I remember this right about the quest with some sick/dying lady. Her bf or what went for some medication and was not returning. I think she asked you to kill her because she was in a terrible agony. If you did so, her bf showed up with medication. If not and you let her be in her in agony, nothing happened and she suffered probably forever until she died or whatever. After that I quite lost taste in the game and never played again, because it quite reflected how the game was half-baked and inconsistent. It also didn't feel that right. I mean, the game itself.

Diablo 3 - Bad.

Divinity Original Sin - Boring game. Voice Spam (I had to turn of speakers when visiting merchant), Just stopped playing.

EDIT: BG3 - I didn't play it (if it counts), but it's all another broken (and loved) series and a letdown to me.

Just some random ones from of the top of my head. EDIT: Of course I don't play what I know will be a letdown. But these game surprised me with that and I did expect better. Gaming itself and GOG is the biggest letdown to me these days. As I am quiting GOG and most probably modern (non-opensource) gaming altogether from many reasons.