Classic VW bugs famously have an issue with their clutch cable- it will eventually break, causing the clutch to unexpectently engage. If you happened to be waiting at a red light the car could lurch into the intersection if you didn't have your foot strongly on the brake. (this happened to my dad, no injury, just scary).

So the controls have to be reliable and robust. It will be easier to demonstrate this if the system is simple. So I think we would prefer software to not be involved in safety systems, but if it must be done you need to go to extraordinary measures to demonstrate its safety.

How you do this for something like full self driving where there is a trained system involved (i.e., that you don't know how it works) I have no idea.