Ok so, I was of the same opinion of the OP since the beginning but I really came to understand how broke this mechanic is just now that I'm mastering my own campaign (Descent into Avernus btw, thank you BG3! :D)
I can understand giving "advantage" against flanked targets. It is not a strict 5e rule (which uses the "help" action in order to give another player advantage on the attack role) but it is not unbalanced in this game since the party is almost always outnumbered and it's not so easy to always flank a target with two characters. Giving advantage for backstabbing an enemy just with a character alone doesn't really make any sense.

Unless you are hidden, in the middle of a combat an enemy just does not give his back to you to stab, it would face you instead (both in D&D rules and real-life). Even because turning around on your spot doesn't cost any movement point, since at the end of your movement you can pin down your character and make it face any direction he wants. Even without cheesing the broken jump mechanic my characters always have advantage in combat due to this rule. It makes half of the spells in the game completely useless.

The same thing concerning height advantage. I think having a higher position on the battlefield than your enemy should make it easier for you to hit him, but a straight advantage on the roll is just too much. Just give a +1/+2 on the hit roll (or, even better, just a range increase), most of the times it's sufficient to make the difference on a hit or a miss, and you still have a purpose for spells that give you advantage on attack rolls.

I'm sure I didn't say anything new, just want to spend my 2 cents on this.