Kayn is a pretty good champion concept.

I love this champion. He doesn't feel cheesy in that his kits makes the opponent feel like they have no agency. He's not an oppressive early level invade jungler, but in turn unlike Evelynn/Kha'zix/Nocturne he can deal with cancer invade like Udyr/Yi/Xin Zhao/Shaco pretty well due to his E, so he is able to at least rotate somewhere else to try to compensate for the lost camp instead of just dying to an invade from a duelist.

He's flexible and offers adaptability to the team needs. I've made electrocute work on both forms instead of trying to insist on conqueror/pta with red form since you're not a duelist most of the time but rather a pick-maker to catch someone out. He gives up massive early burst and overbearing CC in order to have a fairly healthy clear. His ult while being an effective juking tool/invuln has counterplay in terms of allowing the opponent to reposition favorably and prepare to retaliate.

When I lose to a team with a Kayn it never feels awful like losing to a team with a Shaco/Yi/Jax/Udyr. When I play as Kayn I feel powerful but rarely braindead like a jax point and click dash stun or yi blink invuln deleting 50% of a squishy's (no comment on shaco, the champ needs a nerf to his E badly, either by removing the blink or significantly reducing the duration of the stealth and the duration of the fear on his boxes as well).

Blue Kayn is a fairly done well assassin concept. The big part of his damage relies on landing his W with its delay built in and he's squishy with limitations on using his shadow walk while in combat, so he doesn't feel like an Akali or Zed or Khazix or Shaco who can do damage near instantly and rather safely because of how they don't permit retaliation with their overloaded defensive kit.

Blue Kayn is a high risk high reward form that's admittedly far more skill intensive than the red form, but the red form has power on its own in terms of landing a really good knock up and in general being a great damage sponge to have cooldowns wasted on you as you knock up people to heal from W and your ult to prolong your soaking of resources so your team can more freely go in.

More importantly, his power curves feel right. It's not hideously delayed like the previous iteration of Kayle and Kassadin, Evelynn and Vladimir, but his late game scaling doesn't make your opponent feel like they lost the ticking time bomb game.

More champions going forward should feel like him.