From a tactical turn based game point of view... these mechanics are terrible both for long term challenge and replayability (it could be better with tweaked mechanics but obviously, replayability isn't an issue in BG3).
The whole map is designed arround verticality (which is fine) but the current advantage + disadvantage from highground is a god mode. God mode should never be a basic thing in tactical turn based games, especially when it's so easy to have.
Backstab totally exploit the TB system. It make sense in games that have a cover mechanic because it reward players that use smart strategies and suceed at flanking ennemies. That's not how it work in BG3 and the cost/reward ratio doesn't exist at all. This is a systematic mechanic - not because it's a good choice among other but because it's the only good choice for melee character. On top of that, ennemies aren't smart enough to use it against us and they'll never be smart enough to cover their back.
We should never have such bonuses over AI, everything should have consequences and nothing should be systematic in tactical TB games.
Flanking, the optionnal rule of D&D : - doesn't give any bonus over AI (the AI auto use it) - have consequences (ennemies could have advantage against you) - wouldn't be systematic (depend our party composition - number of ennemies - risk/benefit)
On top of that : - it would increase synergies between characters - it would create lot more choices/tactical decisions in combats (should my ranger go in melee to delete advantage ennemies have against my fighter ? 3 melee ennemies, I only have 2... Should I buff Gayle and go in melee ? Which ennemies am I going to focus on to delete their advantage ? Should I cast a specific spell to delete ennemies advantage ?)
I guess they choose advantages so players can increase their %to hit easily but that's totally contrary to their will to give us lots of choices in the game. We don't have much tactical choices to fight in BG3. We only have a few good choices that have terrible consequences on the whole game balance.
They could simply allow us to stack flat bonuses to increase our %to hit. All this already exist in D&D : +"x" arrows, +"x" for weapons, "x" from bless, +"x" from highground >< +"x AC" from cover,... If everything is reasonable - advantages would still be a thing and - "missing" wouldn't be a reported issue anymore.