Observations from playing Kayn
He's locked into all of his ability animations (which have rather long animations too) which can be really frustrating. His Q is meant to be a gap closer but it's not hard to escape from Kayn by juking it. The range of his Q is also deceptive - the scythe part doesn't actually match the range of the animation, it's actually smaller. So people nicked by the visual scythe may not take damage. Also note - Kayn has absolutely no animation cancels, which are generally a staple of Assassins/Fighters so that they don't feel clunky (So yes, Kayn does feel kind of clunky) The cast range on his ult is really low. Which I find surprising as you're required to tag someone before ulting them. Often times I'll have to flash, or waste Q, to catch up to someone who I want to ult. It also feels like there's a bit of a delay on it. The ult says you can use the ability again to re-activate it, but you can't do it instantly. Once you cast it, you're locked into it. So often times you'll end up in the middle of several people waiting for you re-surface when you ult during Teamfights or skirmishes - with no way out. So much for utility. It's damage while in base form is also pretty bad. His E feels really satisfying to use the more I get used to it. But something that's really annoying is that there isn't a very good indicator as to when you'll be rejected from it. Often times you just are. It needs a better warning, like the screen gradually changing colour or something. Also, it is quite buggy. Sometimes you'll be rejected from it for no apparent reason. Kayn's base form does little damage and has really weak ganks because of this. Yes, your Q lets you come from all kinds of angles which is great. But you might want to consider that early game Kayn effectively has less sticking potential than Shyvana. You have a meager slow on your W, but most of that utility is wasted by the fact that you're locked into casting it, so by the time you move half of the slow or more has already expired. And then blue smite. Your gap closer, again, is gated by being locked into a really long animation. Rhaast and SA are both extremely item dependent, so you have to snowball to function in team fights.
overall though, I'd say he's really fun to play. So I'll be sticking at him. :) Also waiting for that one-buff that will Rek-Sai him, so that I can ride the freelo train!