You have the same problem with a vacuum assist. If the engine stalls, you lose vacuum.
GM has used electric backup hydraulic pumps as well. I toyed with the idea of trying to rig up a system with just the electric backup, but it is only specced to run for a few seconds at a time and they are expensive enough that I didn't want to risk burning it out trying to run it continuously.
On the electric hummvee that I built, I used an electric hydraulic pump to feed the steering and that worked well. The pump was *very* expensive though.
