You think it's hard to hit W and then ult on Kayn? Kayn is so disgustingly easy that he's practically a free win. He farms much better and more reliably than even Zed, who's entire waveclear is predicated on actually sacrificing cooldowns in the short term. Living Shadow has something like a 20-25 second cooldown early in the game and if he uses it for waveclear purposes, he puts himself at risk and diminishes the window he has to actually harass his opponent. Kayn just needs to hit Q, and all of the mana he just used will regenerate back in a couple seconds and the move is on a 4 second cooldown.
The notion that Kayn is some how more difficult to assassinate people than Zed is UTTER nonsense, and i really hope that's fully understood here. Kayn's cooldowns, his speed, and his hitboxes are far more forgiving than Zed's can ever be. Doesn't help that he can walk a quarter of the way across the map, ignoring every obstacle he sees at 600+ ms.
And no, he can't just R auto E and R back with Duskblade. Quit the shit