"People are frustrated because they come to the game with a strong understanding of how 5e works, and then that's changed in a way that they can't make sense of and makes the game less fun for them."
Isn't that the definition of "it's different, so it sucks?" I feel like tons of things from 5e are still here, too. Someone who's never played dnd will still be playing catch-up to the players that recognize all these features, no?
Um, no. It's not people going "it's different so I don't like it", it's a matter of the changes they've made being
bad changes. Cantrips are pretty OP now, being basically turned into free to cast 1st level spells. Effect surfaces are cool and something I actually like, but there are simply too many of them. See they ported the surface effects over from DOS2, and they were awesome in that game, but in DOS all it costs to remove a surface effect is a short cooldown timer on a spell/ability, while here it takes up a limited resource. It's a matter of surfaces being really really easy to apply, and annoyingly difficult to actually get rid of.
The problem isn't that they changed things at all, it's that they messed things up with the changes they made. If these changes were well balanced and explained properly in the game, it wouldn't be a problem. But most of the changes aren't well balanced, and some of them are just plain confusing. For instance, they changed the way that Skill Checks work, not in any major way but just by simply reversing the process. Instead of rolling a d20 and adding your modifiers to the roll, it removes the modifiers from the DC for the check. Not a problem, right? Except they didn't bother to tell us this in the game, so when a skill check DC is 5 and we have a +5 modifier, it looks like it should be an automatic pass but instead if you roll a 4 or lower it fails, making you wonder what happened to your modifiers. I spent a good few minutes scratching my head wondering why the hell it didn't add my modifiers and had to come to the forums to find out that they had changed it to work this way. I certainly would prefer it being swapped back to the way it's usually handled, but that's just a personal preference, since it still works fine then it can be left as is, all it needs is to actually be explained in the game.
Meanwhile, while that's a good change, others aren't so good. And some things they have in the game are just plain not working very well. For instance, there's a few puzzles in there that seem to almost be pulled straight out of Divinity, but the problem is that you can't solve them the same way you could in Divinity. In Divinity, you have cooldown timers instead of spell slots and can have any spells you know ready at all times, but in BG3 you can't have all your spells ready at all times and it costs a spell slot to use them, making these Divinity puzzles tedious to get through because the system it was based to work around isn't present in this game. Don't know why the hell they included the same style puzzles when they should have known they wouldn't work, but then again they seem more interested in slamming 5e and Divinity together to make a flashy game more so than a balanced 5e game that includes some parts of Divinity.
