Firstly the complaint that Larian isn't using D&D RAW and creating house rules. I first played D&D 40 years ago and have played a lot of RPG's since then. The 80's were basically one long RPG session for me. With any system after the fist couple of sessions we always used house rules. That's various different groups. I've never met a group of role-players who didn't use house rules in their games. So why is this a bad thing for Larian to do? Maybe I'm just out of date and all the cool D&D5e kids play RAW now.
Secondly is all the complaints about BG3 just being DOS3. DOS was obviously Larian's way of making a D&D like game in the first place. Party-based, pseudo-medieval with story and narrative elements tied together by many combats. Basically D&D. The rule set might be slightly different, but that's par for the course when designing for a different medium and without a license for the base game. So yeah, it looks a bit like DOS. That's because DOS looks like D&D.
For some it's almost an ideological all or nothing thing, for others like me - it's not necessarily one way or the other, but both. It boils down to what Dexai pointed out; particular implementations. For instance, the Warlock Hex spell implementation is RAW, but also in need of streamlining as the secondary curse ability effect has next to no in-game effect yet waste a lot of time manually choosing attributes (as opposed to automatically debuff all attributes) considering a Warlock cast this spell nearly every round. On the other hand, we have Larian "cheese" like ever-burning candles that puts weapons on fire for a pretty significant damage boost. Or tadpole jump+disengage. Both can be used to pretty much guarantee a significant edge in combat and your patience/boredom level performing the required ritual to leverage this edge is its practical limitation. Or exploitative use of endless healing food items instead of just giving us passive regeneration. It all makes for time-consuming bad gameplay and often lacks internal logic which leads to loss of immersion (esp. the ever-burning normal candles that set swords ablaze or eating 12 apples to heal mortal wounds in seconds).
