I think people are saying "this is unbalanced, or this is unbalanced" because the entire kit is unbalanced, and they are just picking the thing that stands out the most to them.
Some say his ult is like a Zed ulti that heals..
My number 1 problem with ekko, isnt so much his utility, damage or mobliity its the fact that he has all 3. He has a Slow on his passive, Q and W. thats 3 forms of soft CC, then his W also can be a very large AoE hard CC stun. These are characteristics of a tank. (not to mention his huge shield which i think is kinda stupid to have as it doesn't really last long enough to make use) He then has the traits of an assassin, jump in with W-Q-E do as much damage as you can press R and either do a boat load of added damage or get out of a sticky situation with a heal. The ammount of CC that is accessible to him given this damage and mobility is what I consider overtunned.
If ekko is going to be balanced, they need to do a lot of nerfs, similar to that of what happened to rek'sai, She came out with a shit load of damage that was icing on the cake. She has had her numbers nerfed so much on her Q specifically and yet she is still very strong given her mobility (burrow/E) and her utility (underground motion detector/aoe knock up on low CD)
TLDR; Ekko needs to be toned way down.