Unless you're any squishy besides an assassin. Or a tank being deleted by an assassin that got fed before you could even buy armor.
The only fun you might have in an assassin meta is as an assassin, or as a support with a metric fuckton of cc, such as Leona. It sucked ass as everyone else.
ADC? You got bursted instantly, as you'd expect. But you'd be bursted no matter what your team did. Even if it was 5v1, the enemy assassin would still wipe you off the face of the planet, and then it was 50/50 as to whether or not your team could actually kill him before he got away.
Enchanter? Good luck doing anything. You're bursted even faster, and you can't even do your single job of keeping anyone alive. Sure, you had Janna, who theoretically could keep an assassin off. But you'd have to not be bursted from stealth to do that, and you'd have to have your ultimate.
Mage? Good luck even finishing your Zhonya's, let alone actually getting Morello and ganking bot, or winning lane, or doing anything, unless you're against another mage.
Juggernaut? Congratulations, you were the only person who wouldn't die to the enemy assassins, even if they were fed. Assuming you could actually get your Black Cleaver and Deadman's Plate, that is. And you'd never actually get to catch and kill the assassin.
Tank? You were nothing. You'd be bursted instantly by the assassin, since they have so much more gold than you. You never got to be tanky, because the assassin roamed top with his Duskblade, Youmuu's, and Death's Dance, and killed you again and again and again and again and again when you only had a single chain armor and a ruby crystal. Only a couple of tanks could do anything, and those were the ones with tons of CC, such as Leona. Leona had fun, because she could close and keep the assassin still. Although it did her no good, since her entire team was already dead, and she has no damage.
Bruiser? Congratulations! You combine juggernaut with tank! You never get to catch the assassin, and you never get to be durable enough to survive him! Your prize? A demotion, and a report for picking something insanely off-meta.
Skirmisher, you already got reported for picking Yasuo. Although you could actually use assassin items, and you somehow actually did half-decently. Fun fact: That was the only time anyone ever said "Thank god we have Yasuo".
Now, it's possible I'm only saying this because I'm in Bronze. But it sure as hell felt like nobody but assassin mains had fun.