I think it's more the fact that his entire kit is designed to be undodgeable and annoying. He comboes single target damage and disables, has constant movement speed boosts instead of conditional ones, and defuses situations at seemingly random with his ult.
It makes laning against him and fighting against him in general a total pain. It also rewards him for players happening to stumble upon a decision he made 3 minutes ago.
Without just going through the list of abilities and acting marvelled at why they're allowed to exist, I'll just say... Nothing he does is based on player skill. It doesn't give feedback to the player directly on how they're doing. It punishes the enemy laner for existing, not for making a mistake. He directly counters an entire class of champions (melee) and only succeeds in doing so when he is in an isolated lane where his opponent cannot get help easily, forcing the jungler to camp or to ignore the lane, both of which are just unfun game states.
With targeted damage and disability, free extra damage and free mobility, he is absolutely poorly designed and unfair.