Mundo's Ulti

A part of Mundo's issue is that he also gets move speed from the ultimate along with good regen. It almost acts as a get out of jail free card in addition to his W reducing disable duration. That's a problem for lane Mundo as it makes him harder to gank because the CD is so low. Increasing the CD forces him to play more safe, plus it gives more options to fight for the other person when it's down.

Other issue is, prior to this, as long as Mundo could survive a little over a minute in lane without dying post 6, he could stay in lane forever without backing. That is also a problem because you can't shove him out at that point, but he can fight pretty freely when it is up and win trades just because he heals all of it (more like 50%) up every minute and 15 seconds.

His late game is where he is suppose to shine, so reducing his early power isn't bad.