Prior to his rework, Dr. Mundo was a decent tank champion. Now he is way too good at everything else.
What changed for Dr. Mundo?
For the most part, his kit is relatively similar to the way it was before. His Q ability still allows him to throw a cleaver, his W still does AoE (area of effect) damage and his ultimate still lets him heal based on his missing health. His only two abilities that were completely changed were his passive ability and his E ability, although the more impactful change is to his passive.
Dr. Mundo’s new passive ability, called Goes Where He Pleases, gives him a passive shield that negates the first immobilizing ability of an enemy champion and, instead of losing his health, drops a canister nearby that can be picked up by Mundo to restore health or stepped on by an enemy to destroy it. His passive also gives him increased health regeneration based on his maximum health, which is similar to his old one.
Why is this change so overpowered?
This one change is what any tank champion would want to have. The ability to have a permanent spell shield gives any champion a huge boost in power, and on a tank champion, it means that they are much harder to pin down and kill. The harder it is to kill or crowd control the tank, the harder it is to get to the enemy backline to take out their carries.
Dr. Mundo, in particular, benefits heavily from this buff because, on top of his heavy self-healing motif, the spell shield allows him to bypass one instance of crown control. This instance normally happens at the beginning of a fight, so if Mundo wants to run headfirst into an enemy Varus, he has no concern for his ultimate because it will not stop him or his allies.
Being able to be the perfect front line means that he can fill a lot of roles for a team. He can do damage in team fights to help take out opponents, he can engage fights with his ultimate or a rune like Predator, and he can tank damage for his team. The new Dr. Mundo is an unstoppable force that has to be dealt with early in the game to prevent him from reaching a point where he can no longer be handled by his opposing team.