Another down side of the UMP - storing the mags. They won't fit in standard mag pouches. They will fit in the same mag pouches as P90 (also a pain in the ass).
Otherwise though, it's a beautiful gun. But mechbox issues aside, I'd still go for a full stock MP5. Everything is easier to get, and will hold a large battery. And fuck what happens to the mechbox shell. A good reinforced V2 mechbox with an added sorbo pad will last a long time without breaking. Makes the V2 weakness sort of moot IMO.