Considering there is plenty of discussion regarding balance, environment, etc, I think one point that gets lost is that BG3 and 5e are inherently different games.
5e is a collaborative game, where the DM and the party create a shared story and play together, whereas BG3 is a co-op game against an AI.
As a DM I would never make the game difficult for the players by focusing their Wizard constantly, even though arguably they are the largest target. Partially because I perceive the enemies might not do that, but also because that would make the game boring to my players.
Whereas the AI in BG3 has no problem throwing 3 flasks of alchemists fire at Gale putting him out of the fight before he acts. And I am fine with that, because the rest of the party will pick up the slack (particularly true in single player).
So I think since the encounters are different, so should some of the rules, and taking a completely copy of 5e rules would lead to a mediocre game.
Because 5e rules are (generally) simple and are just a tool for the DM to create engaging encounters tailored to the PC's. The DM has the flexibility to make the game engaging
Whereas in BG3 the encounters are tailored for an abstract party, so you need to tailor the PC abilities to fit these situations. The players need flexibility in this case, since the game itself is inflexible (albeit has a lot of choice). Therefore some changes to the game mechanics are needed to make it engaging.
