I noticed in my games is that its really fucking hard to compete with other junglers on base kayn, and this puts me very behind especially now when everythign is about one-shotting. Its more or less impossible to come back if I'm behind.
Darkin is mediocre, when I reach the form its almost every time I'm behind with a bc only and not tanky enough, and most times I will die, when I engage.
The assassin Kayn is kinda bad imo because when I reach it the enemy is always teaming up and its impossible to come out clean without at least trading a kill, and honestly other assassins would do the same job a lot better.
In all forms I think I don't have enough impact to the game after midgame, unless really damn fed. Also its very team dependant which is not a good sign for soloQ or even jungle now. My main problem with him is always the teamfights, because I don't know what should I do.