The main problem with tank Ekko is that he does everything. He has cc, a shield, plenty of base damage, and an "Oops, I fucked up" button. Part of what makes tank Ekko so hard to deal with is that you aren't going to out trade him because his shield is ridiculous. You're not going to outrun him, because the range and width of his q is massive, and if he jumps onto you, he's going to proc his passive, making him super mobile as well.
Not to mention, when he ults, he gets a percentage of his health back based on damage taken in the past 3 seconds, so if you engage and start bursting him down or doing some heavy damage to him, he can just out trade you, ult, and then increase how much health he has left to ridiculous levels, all while your abilities are still on cooldown, so you're essentially fucked.
It's kind of why tank Akali was a thing (being a hyper-mobile champion with loads of damage and the ability to disappear completely). At least, before she was totally eclipsed by tank Ekko.