Ekko's idea was a high mobile assassin, He couldn't fulfill that idea efficiently because his damage wasn't burst worthy especially with his skills be difficult to land/Fulfill the full purpose. One big example being his Q, His Q's second proc is really easy to escape taking away a big piece of his damage. The next example being his W which isn't exactly put in with his damage but it holds the same problem as his Q it's relatively different to land/Use properly. His E fill's that wish by keeping him moving, mobile and makes for a guaranteed piece of damage that pushes the assassin theme. His R offers a lot of options to him, Though it holds the same issue with his Q and W it is difficult to use offensively without being 5 steps ahead of your opponent, Especially with the direct knowledge of where it is going to land that the after-shadow gives you. His Passive gives him the only guaranteed damage alongside his E. Ekko's high burst damage spells which are R being a High risk/High Reward because you have to position it which often means putting yourself at risk to position it where a enemy would walk without ensuring your own death and Q which moves relatively slow making it difficult to get the full damage it offers (Enemy have a obvious time period to walk out of it's second hit.) Those skills being difficult to land/Use properly takes allot away from his options of bursting like a assassin is supposed to. W could fall in the same issue as I stated earlier it's difficult to use properly without thinking ahead of your opponent taking making Medium risk/High reward. Taking a big piece of his utility away. The only reasons why assassin Ekko is as reliable as we want is because majority of his damage is easily countered/brushed away with simple game knowledge. Tank Ekko works well because it takes the High risk/High reward aspect away making it Low risk/High Reward. I don't think he needs nerf per say just tweaks to spell that make them more reliable to use Q being the biggest competitor for this. W and R are debatable. I feel his passive ties in well with his theme and helps him out quite allot. Sorry for the big essay, Just though I would offer my two cents because Ekko has been my main since his release and i'm passionate in his place with league's game play. These are just my opinions and should not be taken as direct fact. 
