I don't see all this as a problem. This is a taxi device. If an aeroplane has a failed engine on start, it can simply taxi under ATC control using this device until it is out of the way and making its way back to the apron. What is the problem? If it fails on push, presumably friction of the motor will provide braking, or automatic safety devices? It will only be pushing back at walking pace- what's the problem? You may as well ban pushbacks in case the towbar fails. You can easily stop gently- just brake very lightly.
I see a problem in getting it out of the apron depressions at the parking spots in the first place!