It's not that it has to be no-fail it's that the failures need to be a subset of the failures that humans make.

If the failures are in places that humans can easily and reliably handle (which is the case now) then people won't trust these systems. If the failures are created by the software not being able to handle basic driving tasks and wouldn't normally happen with a person driving then this is a huge problem with the system. A system that repeatedly drives at a lane divider repeatedly is not something people should trust.

https://arstechnica.com/cars/2019/03/dashcam-video-shows-tes...