@acatlas, you seem to be arguing that
1.) Larian's implementation of Advantage and Disadvantage match tabletop, where you roll twice and take the higher or lower.
2.) It makes sense that it is easier to hit someone from high ground, and thus you should get Advantage (capital-A Advantage, instead of just an advantage)
3.) It makes sense that flanking a creature would grant Advantage, and thus Larian's backstab is fine.

My response
1.) No one is arguing that Larian's implementation of these mechanics differs from tabletop rules.

2.) There is a difference between capital-A Advantage and "an advantage." Most of us aren't arguing that it is easier to hit people from high ground, but that getting Advantage is too powerful for something that is so easy to get (walk or jump up a hill). It invalidates a lot of the other methods of getting Advantage (spells, class abilities), since sources of Advantage don't stack. If this bonus was changed into a +1 or +2, then it would be more reasonable (less powerful) but also not invalidate all those other sources of advantage.

3.) I agree that it makes sense that flanking could grant Advantage. However, this is NOT what is happening in BG3. In BG3, you get Advantage for simply moving behind someone, regardless if you have an ally on the opposite side. This doesn't make sense, because in a real fight that enemy would turn to face you.

Originally Posted by acatlas
Modifying the mechanic to be +1 or +2 just completely defeats the purpose of the games existing rule set
Why do you say this? D&D 5e already has a system that grants +2 or +5 to enemy AC (the Cover system) and +1-4 to attack rolls (Bless), so using flat bonuses instead of advantage is not inconsistent with the game's rule set.