The main complaint I've heard is the new grapple rules (because it's always the grapple rules) make PC grapplers non-viable
Well alright, I don't care about grappling. Let me introduce you to some of my main complaints:
1. I think the whole edition was premised on a wrongheaded view of simplicity. It is now a simpler game in a top down sense of using the same keywords whatever, but it is now a harder game to get started with because
everything plugs you straight into whole mini-systems of things (ie: feats). It is easier to get system mastery now, but there are larger barriers to entry to just play a low level character.
2. In the pursuit of "balance", "simplicity" or whatever player options, spells, etc. have lost a lot of uniqueness in favor of copy-paste mechanics. Everything feels samey.
3. Using the same established name for most spells, abilities, feats, etc, and having them mostly be mostly the same but then having a few things be completely different is really just creating an obnoxious minefield for 5e veterans.
4. Insisting on 2024 D&D also being "5e" has now made googling a spell or monster for either harder, slowing down game tables around the world.
5. I despise the culling of half-elves and half-orcs. It's certainly ironic that an effort to avoid sensitivities around concepts of "race" has ultimately ended with an enforcement of racial purity.
6. Doubling the output of the basic healing spells makes healing less precious and players less likely to venture forth not a full health. It also means that if a table is mixing and matching spells between editions they have to have a conversation about which healing spells they are using.
7. Free heroic inspiration abilities undermine what inspiration was supposed to be.
8. I think weapon mastery properties add complication without adding much real value and are just a thing martial characters get. If they were unlocked by feats such that players who actually wanted to commit to them had them I'd consider them pretty cool (though I think some weapons clearly have the wrong properties), but as is it's just one more fiddly bit I have to master to DM a game.
9. People seem to fawn over the new style of conjuring and summoning spell where you summon a spirit with a stat-block next to the spell instead of pulling something out of the monster manual. I hate it. I don't like the level of abstraction between the fake thing that only exists in the spell and the fake thing that is supposed to actually exist in the fantasy world that all this "spirit" nonsense evokes. I didn't really like the 5e summonses because I felt they usually (with a few exceptions like your Summon Greater Demon demon going on the loose) brought in a mindless, personality-less sack of hitpoints when summoning should involve bringing in a being from the fictional universe with personality and consequences, but 5.5 has just gone further from what I want.
10. Generally many of the objective improvements there are aren't really innovations for those already using the Tasha's optional class features, and common, basic homebrew rules. In other words I'm not giving them credit for making drinking a potion a bonus action; most people were already playing that way.
Now I'm not going to not play 2024 D&D or anything. If that's what the table wants that's what I'll play. None of it ruins the game for me. But basically from my perspective they made my favorite game about 5% worse and made me buy a bunch of books and spend dozens of hours carefully digesting them in order to parse out how it works if I want to keep being able to freely play and DM at any "5e" table. Even if I considered it five percent better that would be deeply obnoxious to me.