Dr. Mundo Is Not the Best At His Own Niche: Regeneration
I mean, sure, his ultimate does heal him for a decent bit, but when you look at the exact numbers, there are many champions who on average heal themselves for a ton more than Dr. Mundo and also provide other benefits.
http://www.champion.gg/statistics/#?sortBy=general.totalHeal&order=descend
Based on these statistics, an average Dr. Mundo heals for a grand total of 10,034 health over the course of a game. Other champions that provide only self-healing that heal for more grand total health include (and excluding Junglers due to Smite bonus healing): Swain- 10635 Total Health Mordekaiser- 10709 Total Health (can also heal allies, but not notably, slight cheating) Trundle- 10763 Total Health Maokai- 11497 Total Health Sion- 12578 Total Health (cheating a little since its from passive mainly) Vladimir- 19876 Total Health Honorable mention to Zac in the Jungle who heals for a grand total of 27481 Total Health on average!
So, why is this bad/important? Well, why do you pick Dr. Mundo? To heal yourself! You pick him to go where you please since you know and hope any damage you take will be healed back and any CC that hits you will be reduced through Burning Agony. So, if there are all these other champions who are healing for notably more health on average each game, especially Vladimir, why would you even pick Dr. Mundo. This isn't a matter of balance, it's a matter of identity. Dr. Mundo is outclassed in his own niche! This isn't to say he needs straight buffs or that ANY of these other champions need nerfs, but it should clear that the real reason anyone would be drawn to Dr. Mundo is underperforming currently. Some might argue you pick him just for his insane Q poke potential, but that's not a FUN reason and is done by many others! Dr. Mundo's healing is just in a sad state right now. He needs help that might be something beyond just normal balance changes.