Mundo is so boring
I've played Mundo off and on since I started league. I love his theme and the fantasy of being an unkillable regen tank, but time and again I am turned off of him by how insanely monotonous he is to play.
His passive is completely non-interactive and BAD to be honest. It does next to nothing early game, negates health costs mid game, and finally kicks in late game when it isn't necessary because Sadism just overlaps it during team fights.
Infected Cleaver is actually a cool spell (and not just because it's a skillshot). Lets Mundo farm from range with some effort, acts as a good chasing tool, but it's his only source of damage. His other two basic skills are just farming tools that are useless late game because they have no consistent scaling.
This brings me to my main point. His skills do decent damage in lane while they're still scaling with level, but he does absolutely no damage late game, which is supposed to be his time to shine. I'm not saying all champs need damage, but he also lacks any hard cc. He just stands around in team fights, maybe tries to stick to a carry. It feels really shitty. You can't do anything.
tl;dr: Mundo is a slow farmer who takes forever to ramp up. As you watch your health bar increase, you also watch your damage decrease. Playing him feels useless and binary.

and a lot of other old fighters, in that he's designed to be very good at tanky DPS, but wasn't really designed with a strong gameplay fantasy in mind (he's a ball of stats). All of his crazy tankiness and healing doesn't really help him as much as it should, because nothing he does to his opponents is strong enough to really draw attention to himself: bar a mediocre AP scaling to his W, there's also no real way of scaling his damage, so the only way of making his kit scale is to stack health. With the huge amount of straight-up tankiness and regen, though, there's also no clear way of killing him once he really gets rolling, bar hard-countering his ult with Grievous Wounds.
all he needs.
his w, cinderhulk, q, e, smite, that's all he needs to blast a squishy down.
and maybe a pair of boots (
,
, and
can all be great choices).