So can anyone explain how Ekko is more of an assassin than before?
The whole problem with using him as an assassin is his full damage combo takes about 4 seconds to complete after you engage. His parallel convergence takes an additional 3 seconds. Which means you're engagement is going to be around 5-7 seconds in which one of your main damaging attacks is delayed until the end of that.
That seems to be the main problem, Ekko's R is almost always best reserved for a disengage instead of actually attacking the target. Whether they like it or not, they've kitted this guy to be a tank.