Depends which version your talking about
Full tank ekko is pretty brainless if you ask me, for you simply become this annoying guy can stick onto anyone who can now easier land his stuns for people will be piling up on you and you can now last for a long period of time.
Full AP ekko takes a lot of skill though , you have to keep track of where your ult is, you have to use your W in a very pre planned manner when ganking other lanes, and since your squishy, you now have to pick your target very wisely with your E, because if you chose the wrong person or approached the situation incorrectly your just gonna get blown up.
Tank Ekko does have weakneses but the main weakness is, that it scales horribly late game because at this point Legitmate tanks are tankier than you, and enemy team carries can now kill you for they caught up in items.