He doesnt have ton of waveclear or AoE damage. He cannot get tanky by building tankier items if he gets ahead, he will always be squishy and be able to be targetted by the enemy team.
He cannot 100-0 someone unless they are either A. Alone and squishy or B. He is super fed and lands his whole kit.
Lastly, even if he is fed, he still has a hard time diving into a large fight to target 1 champ, such as an ADC, support or mage. He needs to wait for the right time when he can go in and begin to use his ult execute to start changing a fight. Also, his engage is his disengage. So once he commits, he can't skip out 2 seconds later, giving his enemies the ability to react.
Overall as an assassin, Pyke fits what an assassin should be better than any other assassin.