> Masochism (E)
> max bonus AD lowered from 60/100/140/180/220 to 60/90/120/150/180
> minimum bonus AD lowered from 30/50/70/90/110 to 30/45/60/75/90
Mundo's E damage first got buffed because he was ignorable by ADCs later in the game and also struggled in the jungle. He spiked mid-game, like most tanky champions and then fell off hard once itemization kicked in (penetration, BoRK, Liandry's, Black Cleaver etc.). He was supposed to soak damage but was really just more or less ignored in teamfights and ADCs could easily kite him and outheal his damage. His play pattern wasn't functioning well. He doesn't offer hard CC, an engage ultimate like Malphite, or really anything else.
Going back to that isn't ideal and this change also reduces his power when very low health and going for some clutch last hit on a champion. Likewise, a Mundo who plays around his HP bar well and tries to optimize his damage with the perfect R timings etc. will not get as rewarded. I am fine with the minimum bonus AD being lowered but I don't think the max AD bonus should be lowered as well. Let him have his high moments. What can be done perhaps is to make the bonus scale differently at lower HP (requiring more risk to get the max bonus).
Right now, Mundo is an attractive pick in SQ and competitive play (when Camille & Irelia are not up) but his performance isn't dominant. He is safe, a good meat shield and a good duelist later on. In SQ, his win rate this patch is 50.21% for JG and 51.28% for Top. In Diamond+, that drops to 48% JG and 50% for Top. While he's supposed to be good late game, he's only ~50% past 40 minutes. These are acceptable win rates and are not out of line. They're also not depressed by random players like for Yasuo, Lee, Irelia or similar. Champions with lower skill floors are expected to perform at least to 50% even when not overly strong.
Mundo is not as easy to play optimally as it seems since you need to land a spammable skill shot, auto reset, optimize HP ranges and know your limits as well as what you can get away with macro-wise but his skill floor is quite low. He's a fairly predictable champion for both sides. Thus, I think the win rates are representative of his power level.
In many ways, he tends to be a "win-more" champion. If behind, he doesn't frontline well or have enough threat so he tends to be more or less out of the game if he can't get his stats on time. When ahead, he seems nearly unstoppable and unkillable. Objectively, I think this nerf is too harash and he needs to be riskier, not necessarily weaker.
BTW: It's also notable that comps. are AP heavier than usual. Vladimir Bot, Brand bot etc. are all going to make Mundo indirectly stronger since he is good into magic damage. Thus, he SHOULD be stronger right now. ADCs are also turning on a tad later than normal so again, this favors Mundo because ADCs are the main threat to him. If snowballing is reduced, ADCs will become more favored again and the games will be a bit longer too so I think the meta will slowly turn against him anyway.