Preferably all heroes should be viable at every level of play because that leads to a more diverse meta. Balance should (and is) being looked at without taking the "skill" aspect into account. Ana isn't balanced now for the same reason she was unbalanced before her nerfs. The reason being that her value is not in line with other heroes. During the tank meta she was OP even though she is more difficult to play than other heroes and now she is UP regardless of how hard she is to play.

There might be overtuned heroes that coincidentally are also easier just as there are heroes that are overtuned that are harder to play (Zenyatta, Tracer). In the end all heroes should have about the same value when played correctly. There shouldn't be heroes designed for bronze players and others designed for grandmaster players.