Mundo does need a nerf.
A good Mundo and Singed can run around all day and not get ganked unless its 3v1.
Any hero played properly can be a pain to kill.
Tanks are a counter to dps carries. If Singed, Cho, Mundo, etc. are nerfed, I expect to see a subsequent nerf to twitch, evelynn, Ashe, and every other DPS carry.