Kayn and the problem with Assassins/Marksmen currently
Riot has taken the design philosophy that assassins should not be able to quickly kill opponents under normal circumstances. In this meta, if everything goes according to plan, it should take approximately 2-4 seconds for a healthy assassin to eliminate a low %health ADC or Mage. The defining characteristic of an Assassin is the ability to evade those who would stop him/her from killing a target, otherwise known as being slippery.
Kayn's kit exemplifies this philosophy perfectly, boasting incredible evasion abilities with his E and his Ultimate while having his damage focused around 3 second time windows. His whole goal is to flank, attack a target, become invulnerable, disengage, meander around the fight, and re-engage when possible. He's great at constantly staying on the move and being difficult to lock down when played properly.
There's just one thing he struggles with: Killing a Fed ADC.
Analyzing the outplay potential that assassins offer, it's clear that they're incredible at fighting Spellcasters. Champions like Kayn and Zed are able to threaten the likes of Syndra and Lux because they have the proper tools to avoid skillshot abilities. It's a satisfying experience because of the high risk that you might straight up die if you misplay an all in.
However, Marksmen have almost no counterplay to their damage patters; they auto-attack you, and your ability to succeed is dependent entirely on stat-checks and the chance of a critical strike. So when I dive onto a Twitch, it's likely that he'll have enough lifesteal to survive my delayed burst, or just straight outdamage me entirely if he has the correct items. The worst part is that a successful kill in this situation isn't even satisfying due to everything I just stated.
It also doesn't help that every new item that gets added only increases the untouchablity of Marksmen.
Wasn't there a time where they were supposed to survive based on positioning and the skill of their Support? Now they can completely negate any Assassin's damage with just a few of these items. Hell, it's not very unlikely to see ALL of these items used in a single game.
In Conclusion, Assassins are really weak because of how safe the ADC role is right now. Marksmen are more powerful than they ever have been, and it's nearly impossible to assassinate one that's popped off. It's too easy to keep an ADC from dying because of itemization, and certain Marksmen are just overpowered. I'd start by nerfing Phantom Dancer honestly, but that's just my opinion.