For a champion that is supposed to be an Assassin, Ekko requires too much guessing
An Assassin should sure he/she can 100-0 most squishy champions, that's their role. But with Ekko, you try to ware people down (emphasis on try) and burst them with your E, or hope that your ult lands. You have to guess where you opponent is gonna be to get the most out of his W. You have guess where the enemy is gonna go for you to burst using your ult, or use Zhonyas to make his ult more practical. This is mainly why Tank Ekko is so popular, because his AP build is so unreliable for most people to play effectively.
An alternative example of what assassins should be are in champions like LB, Zed, hell even Fizz can assassinate better than Ekko. The whole "utility assassin" thing was a failed experiment.