Google still has a human driver onboard when the computer "needs" help. We do not know how often the human has to step in.
About hardware failure, how long do your PCs or laptops last? And when they fail, what happens?
If the computer in your car fails, is it going to accelerate hard, or stomp on the brakes? Does it swerve hard left or right? Keep coasting straight at the same speed? Or is it going to be different and unpredictable for every failure? I guess eventually the car will stop when encountering sufficient obstacles.
I worked on a fault-tolerant autoland autopilot 35 years ago. Then as is now, the only way to handle failures is to have a triplex, sometimes quadruplex system, and use majority voting. This means 3 computers to fly the plane while only one is really needed. If one channel says the plane should go down, while 2 others say the plane should go up, the majority wins. The failure is logged, and when the aircraft lands it is not permitted to fly again until the problem is fixed.
This redundancy is carried throughout the plane. Even the cockpit switches and dials are redundant (2 or 3 built into 1), so that a bad button will not cause a wrong input without being detected. Actuators to move the plane control surfaces are triplex, so that two good ones will override the bad one. The control surfaces are often redundant themselves. Jetliners like the DC-10 have upper and lower rudders, inboard and outboard ailerons, left and right elevators, etc... Ditto for triplex hydraulic systems, electrical power sources, etc... Heck, we used to have 3 human pilots in the cockpit, and still have 2 today. Redundancy, redundancy...
Granted, car systems do not have to be so elaborate, but certain safety aspects are still needed and must be defined. In the wake of the Toyota infamous "unintended acceleration", the NHTSA has been criticized as not being as knowledgeable in regulating auto electronics, to the same level as the FAA in establishing safety standards for airplanes.
"Old age is the most unexpected of all things that can happen to a man" -- Leo Tolstoy