As someone who plays a lot of twitch, he is not "op".
He is however over tuned with a kit that specializes in catching opponents off guard. It is a combination that is generally unhealthy for any game.
A lot of people say good damage is too high early, bit he has a weak lane. They let him stack fervor and poison without realizing all his early damage is a 70-90 mana cost spell on a 300 mana pool.
They say he is op mid because he can catch you out, but any champion can catch you out if you don't account for their kit and vision you would need to deal with it (which is harder still with stealth champs.)
They say his late game is op, bit his damage is no higher than any other immobile hyper carry. He simply has 5 seconds of massive damage that feels unfair because of the long range from ult and q.
Against people who know how to play against twitch, he is average; bit he was under-represented for a long time and everyone forgot how to deal with him, so when he came back into the meta, he felt overwhelming because everyone forgot how to deal with him.
Bully him early, if you don't gave good long engage, poke trades and don't let him get more than 3 autos/4 stacks of poison and he won't have much damage. If he's behind he does nothing.
Mid game don't split off to a side lane alone if you can't 1v1 him.
Late game having bruisers dive him make it so he can't hit your backline. It can solo win a team fight if you can pull his ult off the back line.