Kayn is literally exactly what I expected.
A hypermobile, snowballing "get fed or be useless" assassin that can make flashy LCS plays but cause a yasuo-level banrate in any other ELO, with a lot of % health damage, and of course untargetability.
Yes his kit looks pretty sick to play but it just seems like every champion is essentially these complicated mechanics strung together and that no simpler champion can ever be created. Rakan is probably the simplest champion made in the last year... Just too much stuff in a single kit.
Just my opinion.