Never have I thought to myself when playing Mundo "I'm prone to being bursted by magic damage."
This change just feels lazy, poorly thought out and could potentially serve as a lethal change to certain team-comps (AP heavy comps). While I'm not necessarily opposed to that, don't we already have a bunch of anti-AP champions and spells? How does this actually help Mundo? I think Mundo might even be low key broken right now but that's neither here nor there because I have very little to back that claim, just intuition.
However, this does nothing to actually meaningfully change Mundo for the good. Why don't you give his ult some new concept like grevious wounds resistance or a cleansing of grevious wounds if he's able to land his cleaver or something. That's honestly mundos biggest issue, you buy EC and Morelos and Mundo doesn't really have an ult anymore. Give him a meaningful way to deal with that while you including some form of "mini game" with his cleavers.
.
and ruin Mundo's life. AP's only option for GW, 
, only does that when the enemy is under 35% health, and Mundo is almost impossible for an AP to bring down under 35% (at least when alone).
, but many of them have superior sustained damage and ability to deal with tanks and bruisers. Almost all AP champions fare poorly against tanks and bruisers, save a few mages like 
or 
. They also have less options to deal with tanks.
which is mediocre on most AP champions and has a horrible build path for them. Few AP champions have sustain on their own, compared to AD champions.
and 
, at least they don't automatically fall apart when the enemy buys GW.
: Just a big ball of meat that literally anyone else can pull off without having to rely solely on whether he can outheal people or not.
or 
.
. Mundo can deal with grievous wounds if the enemy team doesn't have overwhelming DPS to back it up; these items more than fit the bill, as do the champs that tend to build them. Whenever Vayne becomes meta, I have to just bench Mundo until the meta changes because her combination of %hp damage on W, %hp damage on Botrk, and her constant repositioning on Q with stealth makes it impossible for me to do my job. Just having Botrk be popular is enough to make me stop playing Mundo, although I will say I very much enjoyed the lethality adc meta we had last patch. Most champs are terrified of being burst down by a Jhin with Duskblade, but not Mundo.
AND 
is dedicating an entire item slot to hurt one champ's ult. On top of that, Grievous Wounds doesn't kneecap Mundo like how people keep on trying to imply. It just means you have to play a little tighter.
