I think the biggest issue with Kayn is...
The fact that before runes reforged, he was a good flex pick in terms of what role he did for the team. He could go darkin or shadow assassin and do fine.
But then rune reforged came out, and now you have to commit to a play style based on the game lobby.
Shadow assassin takes dark harvest/electrocute
Darkin takes conqueror or electrocute.
Darkin pretty much wants conqueror always, having electrocute really hurts his sustain and presence in a fight, as he isn't playing an assassin role anymore. The conqueror nerf also hurt darkin and he never got any minor buffs for it.
so yeah. rune reforged was a mistake. RIP