I'm currently trying to play through BG:EE on the Legacy of Bhaal difficulty and I'm finding it a massive slog and thoroughly unfun, even though the last time I've played it I remember having lots of fun doing it. So what happened?
The 2.5 patch happened—specifically the feature described by the following patch note line:
"Heart of Fury/Legacy of Bhaal should provide AC bonuses to enemies"
The patch note doesn't specify the extent of the of AC bonus granted, but according to some posts I've read it could be as high as -11 AC. Whatever the case, it seems to be a very large bonus.
Now, it is my understanding that the change the patch note describes was actually a bug fix, since the EEs previously erroneously didn't include the AC bonus granted to enemies by the Heart of Fury mode in the original Icewind Dale.
However, since Baldur's Gate and Icewind Dale are two different kinds of games with different design philosophies behind them, I propose that the that the v2.5 Legacy of Bhaal AC bonus is overkill for a lot of the enemies found in BG1. The reason being that BG1 contains a lot of enemies that are already pretty hard to hit on normal difficulty, such as Battle Horrors, Doom Guards, Greater Doppelgangers (especially Durlag's Tower varieties), Invisible Stalkers, etc. When you give these enemies -11 AC (or somewhere along those lines), there will not be any hitter builds left in the game that can reliably hit them (possibly excluding Archers).
Case in point: my team of berserkers all have THAC0 scores in the 0..2 range when using Berserk, which is insane THAC0 for BG1. But even so, it is usually the case that they cannot hit Greater Doppelgangers in Durlag's Tower without rolling a 19 or 20 on their to-hit rolls. Sometimes a roll of 20 is required to hit fairly common enemy types, and that's not
fun. This is really starting to sour my experience with the whole Legacy of Bhaal thing.
What I propose would fit BG1 much better is a modular system that would give higher AC bonuses to enemies who have bad AC by default, and lower AC bonuses to enemies that already have good AC by default. That way Legacy of Bhaal would retain its intended difficulty without turning into a terrible micromanagement slog for hitter builds to go up against low-AC enemy types.