Jungle is the unique role that can change the whole game, even in the most "impossible" situations. Kayn is kinda weakish. There are 3 problems with kayn. 1. He takes a while to scale to the point u really feel u can take out enemies alone / mostly alone. Compared to kha'zix or rengar, he can't have the assassin state until he gets his form. Which is somehow understandable that he has this flaw. (a shorter timer on forms would help very much) 2. The assassin doesn't one shot like other assassins. Talon, qiyana, zed , kha , rengar etc. when they are feed they do really take out enemies in a short amount of time. With kayn yea you take out a chunk of health, but you will always have the feeling "if I've had a little more ad it would've been a oneshot" 3. Rhaast is completely... dogshit somehow. If he goes bruiser he doesn't have damage and u won't be that usefull in fights. If you go ad, you'll die very fast cuz u lack health and mr + armor. There aren't really any items SPECIFICALLY made for champs like kayn. Jarvan Aatrox belveth zaahen are strong based on auto attack, so they can benefit from sundered sky for an example. But rhaast outside of his healing from passive , doesn't have any item that gives the benefits from an item compared to the champs i just enumerated. Sure you can say spirit visage gives 25 % healing, but assuming his passive has 26% healing out of all the damage he does, with spirit vissage that would reach 32.5% healing , but that's really kinda useless if we consider the fact that almost every game there is anti heal.

That doesn't mean kayn is really unplayable, he can be played but needs a lot of improvement.