Because apparently he is not an assassin, rather he is a diver.
Oh but he still has the ability to act like an assassin like Diana does but more damage than her because he is single target, oh and since he is a diver he needs proper DPS over a fight, oh and since he is a jungler he needs CC, oh and since his gapclosing is tied to his ult it needs to be real strong.
And since he is a diver he needs some sorta defensive tool, so his W exists.
So the end result is a diver with good DPS, sky high single target damage, both hard and light CC, and a defensive tool in his W along with a powerful gapcloser, whose entire existance only really hinges on the question " can i delete the enemy carry of choice or not whenever i need to?".
If yes, he is OP.
If no, he is useless garbage.
I seriously dislike feast or famine champs.