i feel like shadow kayn q should also have an effect, not more damage or damage amplification but invulnerability. all of shadow kayn damage is put on "W" while "Q" offers very little, giving cc immunity on his e is fine but i think shadow kayn needs a bit more slipperiness for an assassin, i think giving his "Q" invulnerability frames (and maybe a q reset on kill) will help remedy that and kind of fits with his shadow theme.
83k mastery point Kayn player here!
I got this almost exclusively playing Shadow Kayn, I've played Rhaast maybe 8-10 times (I could just never get Rhaast to work).
Personally, I think Shadow Kayn is pretty strong how he is, even in a meta where assassins are weak. Using your E to find and instagib isolated squishies is just so satisfying to me. Seriously, if you get ahead in the early game, one W+Q combo is enough to instagib squishies by 25 minutes. If there's ONE thing I would change about Shadow Kayn's kit, it would be to remove the little pause at the end of his Q in Shadow form, it just feels super clunky.
Both versions could stand to be differentiated further from the base kit. They both have an upgraded W, while they differ between each other getting an upgrade to either E or Q. Really both should get changes to both skills.
Q (Shadow Assassin): Kayn can move immediately after finishing his dash, as the slashing attack comes out. W can be cast at any point during Q's execution, but other skills and attacks can not.
E (Rhast): Rhast gains a shield equal to half the heal amount when entering terrain which lasts for up to 2 seconds after leaving the terrain
On the contrary, I like shadow kayn a lot more than rhaast.
Rhaast is so weird because to heal more and do more %max hp you need to go full damage, which leaves you extremely weak to burst. Just being this super awkward with no gapclosing frontliner who cant really tank and has mediocre damage.
When I play shadow kayn its a different story, running super fast through walls and literally killing any non-tank with a W and an autoattack with 
Shadow kayn i feel is the only one to pick
its not cc immunity, its slow, also, the Q still does nice dmg, because of the W delayed dmg you can Q and both will hit simultaneously
Then why is it most higher tier Kayn players use Shadow Assassin. [sg-lulu]
I have yet to play kayn, but from what I have seen Shadow Kayn is just a harder playstyle to pull off. High Skillcap and all.
Eh. It's balanced out well enough with skill effects. W is a general ability both forms can use; Poke, engage, finishing... It makes sense to amplify it both. From a lore perspective too, its power is harnessed by both Kayn and Rhaast. Q is... just a regular scythe slash. That's the truth of it. Hence why Kayn doesn't gain anything from it. Not only that, SA is focused on being on the sidelines instead of the frontline. E, well, Kayn taught Rhaast how to do it basically, and it makes much more sense for Kayn to have this amplified.
Probably because youre hitting the wrong targets.
Shadow Kayn does nothing to tanks and bruisers. Shadow Kayns only targets should be Assassins, ADCs, and fragile mages/supports. Everything else is off limits to you.
Rhaast does damage to tanks and bruisers, but cant delete squishies like Shadow can,
