So I just played against Kayn, he seems quite alright. He did damage I was expecting, his ganks were surprisingly strong, but that ultimate bothers me. In of itself it is not bad, but he can stay in the ult for as long as he desires. I don't like that.
With Zed R and Yi Q, the time they are untargetable is fixed, meaning you can expect them to get out of their ability and then try to dodge or reengage on them. Even Fizz E you can expect him to land at a certain time, to dodge it or reengage on him. Kayn on the other hand has so much room to recast the ability and you can't predict that easily when he is comming back.
Yi and Zed players already use their abilities to gapclose or dodge, not primarely for damage. And you know you wont throw CC at them or use your escape immediately, you wait until they used their abilities and then dodge/reengage. And Kayn's R is even advertised to be used exactly that way: As a gapcloser or dodge, not primarely for damage. So why does he get the luxury to make the enemies lives even harder than Zed and Yi can?