It shouldn't be as emergency stops can be performed at any point in time - usually when there's an emergency. Obviously it's important that following drivers have left enough space so that they can react in time, but that's down to their driving skill (or lack of, if they're too close).

I'd consider that autonomous vehicles should apply the brakes if they find themselves in an uncertain situation as reducing speed is going to minimise any impact. However, this was being driven acoustically and the handbrake shouldn't have been applied automatically by the power running out. At the worst, if a handbrake fails and can't be used, you can always shove a couple of bricks under the wheels to stop the vehicle rolling.