Talk:Kai'Sa/@comment-5285937-20180412035631/@comment-7709681-20180416160831

Supposedly the stealth in both is meant for safe repositioning, that they can relocate without being targeted, but considering that Vayne's stealth is twice as long as Kai'Sa, is much more spammable in a fair time gap (the R's CD) rather than an unlock that may last indefinitely, and doesn't disable her basic attacks (which still sounds counter-productive for an ADC that's meant for DPS, unlike Jhin) and is a dash you'd ask yourself if RiotJag and co-workers were thinking right.

Now, thematically speaking, I'd guess that Vayne somehow blends with the darkness to go unseen (she's still LoL's Batman, except more noticeably sociopathic) and Kai'Sa's suit somehow has the ability to camouflage her with their surroundings (she isn't rift-walking, that's her R)... notice the use of "somehow", because there's no clear explanation anywhere, so right now the best answer may aswell be "Riot thought it was cool, and if we're making another Vayne may aswell copy all of her".

As for how healthy is in-combat stealth for an ADC, consider again the purpose (safe repositioning) so if the idea is to go untargeted then Xayah would be unhealthy because of her ult.