I think what makes him really viable in tank is that his ratios are strong and don't really punish him for shooting off into non-damage-centric builds. Furthermore, he possesses an overwhelming amount of utility as Assassins go that mixes between offense and defense, and he has great room for clever and clutch plays alike. All in all, Ekko has a lot of power going for him and he doesn't really pay for losing damage the way, say, Riven does. He has good synergy with most items he's eligible to build and is in a unique paradigm where he covers for the fact that he's melee fairly closely.
Now I'm not really into the high-level ranked scene, but I think Ekko will always be oppressive. The slow removal is good, but does he still get a move speed boost? Because that's still going to serve most of the same purpose-- he might not be as convenient to teammates, anymore.