http://grabilla.com/0611b-2a23ff12-92ec-4a66-8263-3782467b7672.png
Hi, Amumu main here.
I have been asking myself this for quite awhile especially after the recent nerfs to his ult; his ult can be reduced by tenacity and I have felt this change quite a bit when peeling off tank since my ult simply isn't as effective anymore. I was hoping he would get some sort of compensation but I guess since not many people play him Riot didn't really care.
Honestly his mana pool mid-late game is not at lvl 16 866 mana. This means that his spells per rotation at lvl 16 with Q/E/R maxed is
Ult - 200
Bandage toss - 120 x 2
Tantrum - 35 x 4
This is not even adding his despair to the spell rotation, this means he'll be OOM after a teamfight usually and that's acceptable. Usually this forces Amumu mains to buy
for that extra 400 Mana and he'll have a few more bandage tosses. \
I have for the longest time thought that Amumu's mana costs, like Sejuani's, should be reduced the more ranks put into the ability. This would simply mean that his early game would be sub par like it currently his but mid-late game would excel. I don't agree that he should have to take the blue from his mid laner to not go oom after a teamfight.