Is their skill in instagibbing someone with no counterplay or options?
Squishies always have options to prevent getting burst.
1.) Vision is solution number one. If you can see them getting into a position to flank you, you can either get in the middle of your team for protection, or your team can use that info to your advantage by baiting them in and collapsing on them as a team. Building strategies around vision = counterplay.
2.) If you're a squishy and you don't know where the enemy assassin is -- all you know is that they can 1 shot you -- then play more safe by not overextending and ping your team to be on guard. Being aware of enemy threats = options.
3.) You can also outplay them if you're better than they are, but that's more of an "improve your skill" kind of a solution.
Assassins have to dive into your team and all-in in order to get their job done. If they fail, then they're most likely dead. If you insta-CC them then they're dead anyway, so what's the counterplay for them? You're effectively making them completely useless. Regardless, unless grossly fed, they will not dive your entire team 1v5 to kill you (the squishy).The only way they can and get away with it is if your teammates ignore them completely. Assassins are highly susceptible to CC and are shit in teamfights, so all they can do is flank/dive the carry. If you deny them access, they're useless.
Instant & targetable CC is a weak excuse for counterplay, which encourages no skill development. It's an "easy way out" for playing around the threat of getting burst.
*I have played zoe more than any other champ since she got released, so I know what it's like to be vulnerable to assassins bursting me. You need to learn how to deal with them. The solution isnt to give THEM no counterplay. To give an example,
In lane, zed destroys zoe. I absoluteley hate going against him. Pre 6 i can poke him down fairly well but post 6, if i get near him, he can all-in and 1 shot me with a full combo. So what can i do? Stay out of his threat radius & farm safe, poke him down when i can, follow him when he roams and counter gank him when he tries to. If at some point he gets close enough to ult and kill me, that's my own fault for being careless and overextending.
Outside of lane phase, I stay close to my team and always try to be aware of where the zed is. I dont run around in the fog of war carelessly. In team fights, I want to be as far away as possible to abuse my range. Im also off to the side to get clear shots with my abilities. So since I'm not always behind my teammates, I'm extremely vulnerable to zed unless I know where he is or where he will try to go. That's why using wards is such an important solution to assassins in particular.