Now I'll admit I don't have TOO many games on him, I believe I'm sitting at 25 games, at something like a 58% winrate.
So Kayn has some weaknesses yes, but I find that Rhaast is WAY too good. I mean you can exclusively build him AD and just heal any damage with your passive and deaths dance. Don't forget that if you do build him AD your Q will do something like 220 + 18% Max health PER HIT and his ult does 52% Max health and steals like 35%.
Why does he need SO much % max health damage?