I've seen them in use, well the industrial ones. They are mostly used as part of a rescue system or to haul / lowering equipment down a structure, mast or shaft rather than a man riding option. If part of a rescue system there is always a secondary traditional rope and pulley system on standby. You can get battery pack or 2 stroke versions.
A bit like using your phone or a GPS for map reading in extreme situations; all if fine until the battery runs out especially when you're only halfway through what you have intended to do. Takes time then to work out where you are on a paper map and that's if you've put one in with a compass in the first place. In a white out condition you may not manage to do so.
Same using a motorised ascender as unless you are carrying additional ascending kit and happy enough to make the change over things could go tits up. They are not the lightest things to carry around either. I quite like keeping things simple, less moving parts and reliance on a power source the better for me