Im unsure what is currently wrong with the current rules out of the complaints of AC class which from what i understand (in my limited knowledge of dnd) is that it effects accuracy to much?
I have to say for the health of the game, and not the table top, the current new rules are better, because it offers exploration and potential for new builds. This is highly important. If you wanted original rules you could just opt for other dnd accurate games. As far as i have seen the current bg3 is fairly perfect out side of a few things.
You see, when I jumped into this EA I knew very little about the source material as well and on first glance thought that this game was doing a pretty ok job. But the more of the criticism I heard and the more I learned about 5e, the more I realized that those 'fun additions' to the base ruleset are only additions in name. Because for every little thing Larian implemented to shake up the 5e ruleset, like weapon dipping or height advantage, they crippled multiple other spells, abilities or other tactical options of 5e that aren't implemented yet and probably can't get implemented in the future without a string of more and more now necessary changes to even retain their usefulness. So you could make the comparison that Larian pretty much dropped like a quarter of the cake to the ground in their attempt to put a few cherries on top of it.