What bothers me about Evelynn isn’t that there’s no counter for her (wards and oracle). There’s a counter or way to deal with practically everything in the game, if you have the right abilities/items/build.
What annoys me about her is that her counter (wards and oracle) and her very presence in a match alters the entire mindset and strategy of the game.
Due to her stealth, any time you can’t see her she automatically has a presence in every lane. You have to play more defensively when facing her, and you have to spend early-game gold to purchase wards and/or oracles to counter her, instead of spending that on regular gear. And it isn’t a choice… if you don’t, you will get ganked and she will get fed.
It’s a whole different game when Evelynn is on the opposing team, and that psychological shift for a single champion is pretty powerful.
I don’t think she’s OP, I don’t think she needs to be nerfed. I just think she’s incredibly annoying.
Though to be fair, I’m speaking as a Twisted Fate player, a champion that is regularly banned in ranked/competitive play for similar global impact reasons. I have no doubt that people find it pretty annoying when they think they’re getting away with a sliver of health, and I ult in and card them in their face.