Here's my take as a 5e DM with over 15 years of experience DMing in general:
-this is likely an unpopular opinion, but random encounters are bad game design and shouldn't be present at all. I've never encountered a player that actually enjoys them. Their purpose in D&D is to dwindle a parties' resources and.. marginally annoy people while the players eat them alive? Having a random encounter where some nameless kobold kills your character isn't satisfying in the least, so random encounters are never designed to be actually challenging. They're strictly there for EXP and resource dumps. Almost no modern RPG includes them because they're -bad- and even worse, boring.
-there are too many magic items. I do agree with this sentiment; they're a dime a dozen. Half of them are entirely useless to your party. I've found a ton of items that affect Bardic Inspiration and yet I don't have a single bard. A good amount of the items you find are to simply account for different parts compositions you're using. A very, very good number of them are overturned and could be toned down.
-resting is and should be easy; however, there should be consequences for resting too often. The irony is BG3 has the perfect explanation as to why you shouldn't rest constantly: there's a tadpole eating your brain. Rather than reducing the amount of supplies you find, it would make more sense and be more engaging if resting meant the tadpole inched ever so much closer to changing your character into a mind flayer... and for that to be a bad thing.
-atat boosting items are awful in general and should never be used. Terrible design decision, enough said
-one thing that's more an issue with 5e than BG3 exclusively is the action economy. This works for and against the player at the same time, depending on the encounter. Depending on the size of the enemy party, you're either going to mop the floor with them (eg, Grym where you have four characters focusing one) or you have 20 enemies at once you get overwhelmed with. Encounter balance is an inherent issue with 5e; one mechanism they use to help address this is Legendary actions/resistances for boss encounters that would otherwise be a total cakewalk. Beyond that, I'm not sure what the solution is other than further limiting actions per turn somehow.
Tl;Dr 5e has issues inherent to the system that hold BG3 back, some of the ones I've seen described aren't actually issues, and solutions for others are rather obvious and should have been present to start with.
