Honestly, the only problem I have with "backstab" (that is to say, advantage on attacks against an enemy that is not facing you) is that you can achieve it without allies, or rather, more than one enemy of the enemy. If two individuals face each other in combat and no allies or enemies are in similarly close range, it's just stupid that you should be able to simply walk around the opponent (or worse, jump over them) and then stab them in the back with advantage. Why would an enemy let you do that? It's just silly. Now, if there's more than one enemy close to an opponent, it makes more sense as they can't keep track of too many at a time.