In my GTE I had a worse failure, the 12V fuse on the battery failed. So it cut out while driving because the DC/DC converter can only supply the average current and as soon as you exceed the limits the DC-DC shuts off immediately. Since the power steering is electric from the 12V supply you only need to demand a lot of torque from that and it'll die. Then you're stuck steering a car which coasts but with no power brakes or power steering, and no hazard lights, or way to wind the electric windows down to flag other motorists ...
Bad design IMO - would be extremely unsafe on a motorway! Luckily the road this was on was not too busy. The car had been warning me to service the 12V system before the failure but I thought I could drive it to the mechanics safely enough. Little did I know...