Oh, quite a lot over the years, but to pick a few I can readily think of:

Cyberpunk 2077: My one big prepurchase the recent few years. And it turned out to be laughably bad. I mean I was expected it to be fairly buggy, but even given that it far outdid even my worst imaginations. Not only was the game insanely unbelievably buggy on release (and frankly it still is, they only patched the actual breaking issues not the constant barrage of weirdness), but it is just... not very good? It has pleny really good components, but the sauce sticking it together is devoid of any design or soul, leaving it to feel like a can of ravioli with too few actual ravioli in it.
Doesn't help that the main quest was, IMO, bad to the point of caricature. At least the handful of amazing secondary quests more than made up for that one. Still, overall one of my biggest disappointments of my 30 years of video gaming, especially in how underdesigned it is even ignoring all the bugs.

Divinity Original Sin 2, specifically co-op: I don't know. This got hyped so much for that particular feature. Yet while the combat moment-to-moment gameplay is hilarious in co-op, following the actual story - basically why I play these kind of games - felt supremely irritating, more so because of how frequently characters get forced into conversations the other player then has to opt into no matter where they were at the time or what they were doing. In a lot of ways I wish co-op would have been more restrictive, to more readily support co-op story consumption.
But it's also weird, because like I said, combat-wise the co-op is amazing. Still, was quite disappointed overall.

Overwatch 2: Feels like a cheap pick, but wow was this a disappointment. Between the dropped PvE, the frankly insulting replacement they're now rolling out for it and their complete unwillingness to acknowledge the switch to 5v5 in hero reworks and balance changes - and hence how half-arsed the entire balance feels - this makes me long for an OW1 clone that really just freezes OW1's state, as clearly trying to modify it didn't work out.

Ultima IX: I don't know how many here are old enough to remember this. It was so hyped. It looked so gorgeous. It was so amazing to see it all in this 3D. And then when it came out, not only could it at best run at glorious 10FPS on my machine (and I had a beast of a PC for the time), it was also buggy and underdeveloped enough that I figure it might just have been CDPR's inspiration for how they worked on CP2077. Plus, in U9's case, there's the extra insult that the story and dialogue is quite inconsistent with the previous games, which was a real head-scratcher. Just a disappointment all around. A really big one.

The Dig: This is a weird one to remember. Because in a lot of ways, I also would say The Dig is one of the best point&click adventures ever made. But it was so bewildering and disappointing to younger me, I just came off the supremely accessible and clever Fate of Atlantis having played it late, and there was so much hype for The Dig, so naturally I got it. I was so disappointed.
Now to be fair, looking back upon it now I can recognize that a mix of my hype and the way FoA went against a ton of industry standard for the time was priming me for said disappointment. It's a good game in a lot of regards, in particular in selling the actually alien vibe of it. But it also has "logic" that would make Sierra Games proud. At least I didn't have to use a necklace on the moon (IIRC) 😂.