I don't play Kayn, and I'm not suggesting anything about him needing a nerf or buff, I just want to know the reason his ultimate is sorta weak. It looks almost funny, him infesting your body, maniacally laughing, scythe hovering over you, about to cleave into your body... and it deals only around 200 damage. [sg-lux-2] Again, not saying it needs a buff, I just want to know why.
It's mainly just to buy him time to wait out enemy abilities (such as AOE ultimates) and to give him time to restore his E cooldown/look for an escape with it.
The Shadow Assassin ULT doesn't deal a lot of damage because it's all packed in his base abilities and passive. ULT is just for stall and execute low HP targets.
Rhaast ULT actually does a lot of damage and scales very well with AD.
Under perfect circumstances(triple Infernal-Baron-Elder and full AD-lethality build) it goes to over 100% of your max HP in physical damage.
He's an ad assassin so he doesn't get a good ultimate. Meme's aside it does make you untargetable so outside of every shielding skill/invulnerability in the game and zhonya's you cant prevent him from doing damage.
It has low base damage because it has extremely high scaling. It's probably to encourage people to build him assassin so he's squishy.
Kayn as in base Kayn, Shadow Assassin or Rhaast?
Base Kayn's ult is weak because all of his power is in his transformation into either Shadow Assassin or Rhaast. It's mainly used to buy time for him in this state.
Shadow Assassin's ult is mainly used to buy time for his cooldowns, avoid incoming damage, and reset his passive so he can burst again.
Rhaast's ult is mainly used to either deal massive damage to a tank, or when you're low to burst heal.
Im going to pretend we are exclusivly talking about pre evolved kayne. Basicaly, Its a strong point and click gap closer with fantastic defensive utility from untargitable. It has lower damage but it provides him windows and time to apple further damage as when using it he can be unharmable as he waits for CDs.
It becomes stronger the moment he evolves into something that fulfills enough of a power spike to make his two forms feel different as a Assassin or Bruiser.
A lot of its power is directly tied to the rest of his kit. It can get him 2.5 CD on all basic abilitys in adition to its damage and gapclosing + PA/Rhast modifiers that overlap synergistically with the rest of his kits evolves.
PA provides % amped Burst after casting because of his Passive. Rhasst Provides max hp % damage and sustain on recast/end of timer.
Both are provide a lot more to him then just flat damage off cast end.
So that's gameplay side. Reality side is video games don't follow real world logic very strictly. There is no reason you have a 
and cosmic cataclysmic event ability's doing less damage then a Fungi some one trodded over 
. Q throws the core of a star? 230 (+ 65%) Base and ratio. His R, Starfire dragon breath (average star temps off a quick google. 4,760 (2,900 Kelvin) and 12,140 (7,000 Kelvin) degrees Fahrenheit)... That ability 16 350(+70%). Satan how ever? 450 + 50% ap. The F**K is that? Also... 
still has one of the most rediclious best case damage ultis in the game and that's nothing but some Angry Birds?
Use reality as a biases and you have to ask who the hell did this? Some traumatized LoZ player with a bad Cuccos Truma?
Good for some low quality jokes. Good for some low quality posts... Not exactly ever going to have a place in a gameplay discussion.
So if you don't get why not all ability's are as dramatic in damage as their appearance... reality is secondary to gameplay. Gameplay reasons are related to the rest of the kit far more then visuals. Darius has many Dunk animations but said dunks can vary in damage. Assume Gameplay mechanics to be part of gameplay reality not the animations or the key the ability is on. Not every champions best skill is their ulti.
Because if offers so much utility.
