Ekko isnt that strong.
People just like to complain a lot.
I didnt really say anything before until i actually got to play him myself. but he is a skirmisher as riot intended him to be. He has Delays on every single one of his abilities. Sure he can burst you if he hits you with everything.
So can Xerath, his q has long delay coming back, his w has a 3 second delay before it can even "potentially" do damage, his e has to dash first before the attack, and his ult detonates to where you were 4 seconds ago. unless your using it to initiate, or just sitting in zonyas, i dont even know how you would survive in a teamfight long enough for your shadow to catch up to you and hit everyone with that ult.
Everyone is making him out to be insanely powerful, but after playing him myself, i realize that champions like, kat, lb, and zed will have no issues outbursting him in 1-2 seconds. he is a slippery champion because of his ult ya. but that just makes him another hard to "kill" mid laner, welcome to the club(fizz, leblanc, zed, ect.)