He is counterable. Annoying, but counterable.
I say this as a person who despises his kit more than Fiora's.
Its always that thing about: "He is really powerful vs melees, but gets destroyed by range." Play a kiter and he has a very hard time. Bait out his ultimate, exhaust him, pick a splitpusher that is anti-AA like Jax or Nasus. Or just ignite him once 3 seconds of his ult have passed,
His kit isn't healthy, yeah, I know. Interactive? Well, the slow is, but not much else. His passive is BS? Totally. Can't lose? He can. Broken? Can be, but many other champions can as well. OP? Nah, more like unfun to go against.