Ive had the UMP for a few years (im using it in my profile pic).
No technical problems, the blowback makes a great noise and it can be long or short depending what you need. I can fold the stock or remove the suppressor for close up or vehicle work.
I only use a 7.4V lipo so cant comment about higher voltages. Ive only ever used the standard hi caps, so have never tried using other mags. The UMP mag is too long for lying prone and obviously you cant fire the SMG on its side in airsoft, as I might with RS.
Its got plenty of rail estate for optics, torches and lasers etc.
The battery goes in through a little door exposed when the stock is swung back, so no dramas there and the hop up us a slider exposed in the ejection port when you bring the bolt back.
All in all, its recommended.