Well I do agree with you that Zac is amasing being that my favorite 3 junglers are Nocturne, Udyr, and Zac. He's not supposed to be a damage dealer and AP zac can literally 1 shot kids. Zac is just supposed to be super tanky and provide nonstop cc and disruption, and his w(especially that u can reduce the cd by picking up blobs now) makes his teamfight damage and sustain pretty good now. When you pick Zac you have to make sure you have a top laner that can deal damage to kill a target (because Zacs whole purpose to to jump around ccing everything and just soaking up damage which he does amasingly, especially with his passive.) You just have to change your playstyle with him. I believe he is viable competitively but only if you pick him in the right situation. Hope this helps.