A lot of things are wrong with Ahri. However, I am not going to discuss this as many are stubborn in believing Ahri is good when she is the worst midlaner from all viable midlaners(yes viable that people consider) .
Ahri's kit is disgusting. Just because you lost to her or saw a good one, Ahri is unviable in higher elo because people know how to play against her. Low elo players for some reason not know how to counter her. I've beated her everytime I went against her ever since 3-4 years ago. That is a problem. Just because I understand her kit fully, that should not make me win everytime.
When I see Ahri in enemy team, I assume free win, or at least 80% chance of win on my side. Why? Because Ahri probably has the most counters in game while having weakest late from APC, weak early, and only average in midgame. People argue Ahri has no counters, but the thing is every champ does counter Ahri in some degree.