Sona still has a number of positive matchups
For a champion with basically no skill curve, the only way for her to be "balanced" would be as a champion who's generally weak most of the time but pulls ahead in certain identifiable match-ups that favor her. If she's just generally a good pick without requiring any effort, then she crowds out the role.
's heals are trash and you are pretty much forced to build full AP. (Sona Player)