Flash and cleanse are great tools for Evelynn, but she also gets a big heal after killing a champion, takes 50% less damage from towers/minions, and her ult (guaranteed to have it after the kill) gives her a movement speed increase if needed to run out of a danger zone and either a) engage the closest champion threat and repeat the process or b) run from any superior threat (usually group) that's nuking you hardcore. I've rarely had to resort to using the ult for a retreat, but with cleanse it's almost a guaranteed escape from any size threat.

A lot of people like to toss around how easily they stamp out Evelynn players by investing gold in using an elixir. That would be true if stealth was the only way to ambush or flank an enemy champion, but as some people have already stated, you don't need to rely on stealth to ambush someone. Plenty of champions use ambush tactics and flanking as part of their strat without the stealth ability. Speaking from my own experience, I like to feel as though I'm contributing something to my team by playing Evelynn when people actually make elixir investments. It's a good thing to make the opposing team waste resources to counter something specific that you don't rely on.

About every third game I play, people pick Eve(or Twitch). And Ive noticed a pattern that she is very squishy and relies on invis too much. As a former Dota player, you learn to recognise dangerous situations like: why is the other guy Im laning against advancing for no apparent reason? or the individual who thinks its okay to push a lane at 1/3 hp. Easy to counter without elixers(ive never bought one) by using teamwork and intelligence.

I personally dont think she's more OP than any other hero that was fed, maybe less so since her abilities scale with AP instead of what you would normally expect from a melee hero. There are many more heroes that need attention over Eve. Maybe revisit Eve after they have been balanced first.

Just because a sight is up or an oracle has been popped doesn't make her defeated just yet. Sight ward or oracle, I always invis in anyways. Because of the stun. It's a huge mistake thinking that simply seeing her is beating her. I just pulled off a triple kill the other day without it, the whole team knew where I was.