I agree completely with the OP on this one. If they want to encourage going for the high ground, just providing a small, static bonus (like a +1 or +2) would be worth it. High ground also seems to grant an increase in the range of attacks, and can be difficult for enemies to reach. Having it grant advantage is just too much, and will absolutely ripple negatively through the entire game for the reasons stated. Backstab is also a problem - implementing flanking instead would be an improvement, but just making it a small static bonus of +1 or +2 would be better (if they want to encourage a backstab).