In what situation would you actually want to pick Kayn?

Honest question here, I've been playing Kayn a ton in ranked and looking at him objectively, I can't think of a situation where you would say "damn, this comp really needs someone like Kayn to fill it out!". Before Kayn transforms, he's pretty lackluster and pretty much can't fight any champions 1v1 in the jungle. His ganks are not strong, and while he can come in from weird places, he lacks the CC and damage to close it out consistently.

Now, his transformations have some redeeming qualities but even then, they are out-shined by other champions that aren't gated by a transformation mechanic. Want a CC bruiser? There are way better champions to pick from than Rhast. Want a mobile burst damage jungle assassin? Khazix does his job better than him without transforming AND can solo dragon, Kayn cannot (at least not without taking serious damage / risk). Want to split push with assassin form? Yi can do that better and faster.

Honestly, Kayn just feels weak and lacking in really solid strengths. Assassin form has high burst damage but lacks escape after he's finished the job, and Rhast is just ok, a decent bruiser with some CC gated behind a very weak untransformed Kayn. So really, when would ever actually want to pick him other than the fact that you just like Kayn as a character and don't mind having weaker tools in order to play him?

IMO untransformed Kayn needs a couple of buffs moreso than his transformations.

Also the transformation mechanic is kind of wonky. It makes you think that building up to whatever form you want by killing melee / ranged makes a difference but in reality everything filling up his transformation bar doesn't matter except for the final kill you get that puts it over the edge. You can kill a bunch of melee guys, then when it's almost full, kill an ADC and magically you can turn into the assassin form. Seems kind of counter-intuitive to me.