From what I've seen from Kai'Sa, here are my little Gameplay thoughts

I think she'll be played ADC more than in the jungle/mid. She's viable as a jungler, but in high elo, expect a ton of invades or an early one. Her level 1 is very weak and she's very easy to all-in prior to getting her E evolved. It's not just level 6.

Kai'Sa mid will also be very easy to zone out or take down due to a weak early and poke supsceptibility. Her 500 range really hurts. Any decent League player would see how easy it is to kill her. You stop her from farming and she's a free kill from level 6.