He has too many issues in his kit.
His his biggest piece of utility being his primary damage spell.
A W that's completely impossible to safe proc in teamfights without an ult and only equals out to average damage when it is proc'd.
A kit that relies on his ult to hit his target when the ult itself can be shutdown by simply having a tank block it's path or dodged using mobility.
A playstyle that requires him to either beable to one shot his opponent or die trying.
An entire spell that does mediocre damage.
And passive that becomes non existent passed level 6.
If he gets ahead in lane he can kill his opponent over and over again. He's an assassin. But passed that his teamfighting phase is horrible because of the number of different ways to counter his ult and the low overall damage of his kit. The low damage makes it more than possible to prevent him from killing anything using exhaust or by stacking shields and heals, at which point he dies.
But if you really want to get down to it, any assassin can do what he does.