Ekko has so much utility and damage that he can be built full tank and still carry hard.
Its just like release Gnar: you can choose to build damage if you are ahead, but if you fall behind or your team demands it, you are just as good if not better building full tank. Ekko's base damage is insane, while his W and Q are such strong zoning tools and follow-ups to allied CC that even if you are feeding 0-10, as long as you keep up in levels and build full tank, you are still a teamfight terror.
