I feel like those Kayn missions are just poorly thought out...
Most especially because someone HAS to play Kayn to get the rewards. And not everyone will be good with Kayn, but every completionists (and there are a lot of those) will want the rewards and try to get them, that would result in A LOT of games being ruined by "that one feeding Kayn", and said Kayn being potentially abused and getting disgusted with LoL, I participated in such games, and that was just a terrible experience for the whole team Kayn was in.
The missions for the VS. event were good, because nobody was pressured into playing, and much less required to pull feats like a double kill or a killing spree, on a champion they did not know or like, and while I do think they were a bit too easy, at least everyone could complete them if they put their mind to it.
Besides, outside of blind pick (that a lot of people despise playing), he gets banned a lot (obviously), and his winrate is really low because of how weak he is, making those mission even less accessible for people who are not pr0 Kayn OTPs.