Assasssins in this game are broken here is why. They designed them in a way in which unlike any other champion they don't just deal damage, they're made for 1 shot combos and furthermore their poor design is made in a way in which they are intentionally made to ignore the defenses of the game. I mean let's take a look at some examples of this shall we?
Yasou's crit, yasou has such massive crit that even if you were to get the singular crit reducing item in the game it won't even make a dent in his 100%+ crit, so basically they designed him in a way to where he hits twice as hard EVERY SINGLE ATTACK after he pushes his crit past it's threshhold.
Vayne, another fine example of a champion that has true damage built into her AA, ironically everything in her combos revolves completely 100% around her AA, so think about that for a second you have a champ who deals literally ALL the damage she gets with 0 resistance because as we know armor doesn't stop true damage.
ekko, not to put too fine a point on it but ekko is what happens when you combine zil with fizz. His landing combo is instant, and lets say you do somehow manage to blast everything you have on him, so what? He'll just reset and your damage would do nothing.
Kat, yes you have to aim her knives, and yes it takes a little bit to master that particular skill but you have to remember that besides for her throwing a knife at you she has the ability to throw it directly up above her and when it lands the damage will be on your head, and lest we forget her abilities give her the option to land DIRECTLY on YOU!!!! DAGGER, MINIONS and ENEMIES, that's what that ability says she can land on. So even if you were inclined to say that her range is ok you have to bare in mind she is infinitely faster at getting to you than anyone else, and for that matter getting back.
Which brings me to my most favorite example of the single most broken assassin in the game bar none, leblanc. Now sure she takes a lot of skill to master, but look at her entire kit, she's super mobile, walls don't even slow her down, her damage is off the charts and if all of that wasn't enough she also makes clones. So LB is what happens when you make a ranged shaco.
Now, it might seem like I'm ragging on assassins "but you should learn to play against them better" right? Well see here's the thing, it's not about how well you play against them, its about that assassins in general have been getting more and more broken because of the direction that rito has taken this game. They've been getting super beefy from all the upgrades while everyone else just gets a slight improvement. So now people who wouldn't generally be 1 shotting you with some abilities do exactly that.