What reason do I have to ever pick Shadow Kayn Or Rhaast?

He's slightly more mobile with being able to move during W; Thats it

Rhaast has hard CC, sustain, TONS of % health damage that shred through carries and tanks, and he's tanky.

There is zero reason to ever pick Shadow Kayn over Rhaast, his burst is meh as he has no way of getting out after ulting, since he wont kill you unless he gets more DPS after his ult. He's incredibly squishy, no good cc.

He doesn't have enough burst, mobility, cc, or anything for him to be worth a pick. For Kayn to be viable he'll need to INSTANT kill you like rengar, or else he's garbage because he brings nothing else.