So you backstab someone that is you attack from the shadows an enmy on their back and you don't have a vantage? Seriously. Lets stay a bit away from the meticoluous argumentation based on rules (but more on personal perspective than objective reasoning), same goes for someone hitting from an higher place (that means they have an obvious advantage both because the bigger area covered by eyesight and difficulty for the attacker).
I get it, D&D, rulkes, book and so on. But the game is designed for a bigger audience, who maybe don't care about a niche set of rules loved by old players.
Also to become king of the hill can be bothersome and noe so easy to accomplish, and the balance of advantage/disadvantage is granted by the fact that as I said is not as easy as it seems to seasoned or skilled players or lucky players to stealth or find an elevation from wich attack (in my own playthrough most of the times my party started battles far from any height or with enemies placed in such way that stealth was impossible).
Again what I think Larian should do is to create different difficulty levels so that seasoned, skilled and lucky (yes luck, I can roll an interminable line of misses and critical misses that sometimes make frustrating playing), can enjoy the effort, and at the same time the other players can enjoy their playthrough.
Also, please a little bit more of flexibility and adaptability?