I'd at least like to mention that, at least when it comes to installing WADs, Multi-Mod Manager seems completely stable. I've had absolutely no issues at all, and that's all I ever use it for. Maybe it's BECAUSE my Wii was essentually stock (just HBC + bootmii) that everything has worked out fine? You know, no cIOS and stuff.
Heck that's why I chose to use it in the first place - it didn't seem to require any cIOS for WAD installations. Nonetheless, I always made a bootmii backup before doing anything with MMM.
Anyway, I'm assuming none of the Wii U owners around here are courageous enough to give MMM a whirl?