It's not about it being his only strength - it's that it's a skill that requires perfect play by opposing team and can singlehandedly win a game while having a 10 second cooldown. It's a catching skill without the risk and the most that he needs is a ward and somebody to kill the pulled enemy with. And there's no reliable way to reverse a pull - once it's done, that's it. Have fun with the next part.
It's not even just that he's toxic to the enemy team - he's old Bard. He can just as easily fuck your own team but pulling an enemy he shouldn't. That's way, way, way too much of a power budget to have on a bloody Q, and even more so since, unlike in other games, he comes with abilities solely built around maximizing his Q use - a speed up to widen distance, a knock-up to hold enemies in place along with a silence to keep them there longer if they're mobile; the only other characters in other MOBA's to compare are Pudge from DoTA2 (whose hook travels further, but far slower) and Stitches from HoTS (whose hook can travel further, but as a default doesn't; also travels slower). Stitches can eat people (and while doing this they take no damage besides the ability's damage) but has that as an optional ultimate - otherwise he has a very brief slow. Pudge has a disable on his ultimate, but it's interrupted by basically anything and everything and already has an item built into the game to basically counter the shit out of him come mid-late game - he's practically useless in any kind of real play.
Blitz is almost instantaneous, comes with a two-second lockdown period on any pulled targets, and scales 100% with all damage (if you've ever seen a Blitz get so fed in botlane that they just start building damage, might as well quit because anybody that gets pulled explodes).