There are criticisms to be made of BG3, but the ones being made in this thread are either flaccid ("it runs out of steam by act 3!", "most players wont even finish it!" - just like every single CRPG ever, and the vast majority of games longer than 30 hours) or bordering on poseur posturing ("its below average in every respect!").
These are not the criticisms being made.
1. My #1 problem with BG3 is the writing. BG3 has nothing to say and does so badly. The main story is bizarre and a nothingburger, requiring immense schisms with established lore to function. The lore is completely disregarded the moment it becomes inconvenient and that's quite frequently, so frequent in fact it starts to grate and becomes impossible to ignore. The villains are ineffectual and sometimes comical in their melodrama. Orin is the worst one, but the Final Fantasy reject isn't much better. The game somehow manages to have not 1, not 2, but 3 psycho serial killers (if you are playing Durge). There is a certain "we can do it better" syndrome permeating the whole thing. Orin and Ghortash are both trying to be Sarevok, at least different aspects of him, but failing miserably, even down to becoming Duke of Baldur's Gate and using Doppelgangers to infiltrate institutions. Shart is obviously Viconia but white and with less character. Viconia and Sarevok are defaced beyond recognition and it feels like arrogance on the part of the writers instead of homage or cameos. The companions are almost all fanfic-tier and that's not an exaggeration or edginess. It honestly feels like the writers have never read anything other than fan fiction in their entire lives. The only one you can squeeze any significance out of is Astarion, seeing as how he is a victim of sexual abuse and that actually affects him quite a lot. Outside of Astarion in a very few cases, companions don't have any agency in their own (inner) lives. Shart changes allegiances at the drop of a hat because you said so. Wyll sacrifices his own father to be rid of Mizora because you said so. This is a symptom of the whole plot and game being written around you, nothing can stop you from doing what you want and it feels extremely artificlal in a bad way. I can go on and on about the writing because it's all bad all the time and it accosts you with it constantly. The cuck squad just aren't good writers even in a general sense.
2. The changes from 5E are almost all for the worse. Almost every single one buffs classes that didn't need buffing or nerfs classes that didn't need nerfing (warlocks, fighters). Ranger is probably the best change because the class was just unplayable and had an identity crisis. This is just a cancerous growth emenating from the "we can do it better" syndrome. This makes the game even easier than it already is and it's a baby's-first-video-game sort of difficulty. I don't know about the custom difficulties, maybe they are much better, but I doubt it improves the AI much. The AI does ridiculous things like summoning a creature
in a Wall of Fire, killing it instantly and then running through the same wall, almost suiciding in the process.
3. The much extolled verisimilitude amounts to cheesing the game, like D:OS1 and 2 before it. There is always a barrel or two you can keep in your pocket in order to cheese an encounter. When it isn't used for cheesing, it's just for filling your inventory with mundane objects. The one neutral aspect of it is how the game is full of verticality. 5E doesn't use verticality very well because all it does is force you to equip ranged weapons on all your characters, just like in Solasta, but it's good for roleplaying and exploration purposes.
4. The music is generic and that's unacceptable in a Baldur's Gate game. There is nothing much to say here.
All of this combines to create the impression that BG3 is the most shameless act of necrophilia in video game history. It's not only average on its own, it has to disfigure a much-beloved series in the process. Its only real virtue is that it sold well enough for everyone to realize TB RPGs sell. Congratulations to Larian.