They only add random failure and the possibility of large scale disasters.
And the latter is the really big danger with autonomous vehicles. The same consistency that means they aren't vulnerable to as much random variation as a human driver could also become a weakness. Yes, a drunk or some idiot on their phone or an unwitting patsy whose drink was spiked at the bar can cause a fatal crash. But they can't simultaneously cause 500,000 fatal crashes by doing the exact same stupid thing 500,000 times at once or by being the victim of the exact same crime 500,000 times at once.
And the latter is the really big danger with autonomous vehicles. The same consistency that means they aren't vulnerable to as much random variation as a human driver could also become a weakness. Yes, a drunk or some idiot on their phone or an unwitting patsy whose drink was spiked at the bar can cause a fatal crash. But they can't simultaneously cause 500,000 fatal crashes by doing the exact same stupid thing 500,000 times at once or by being the victim of the exact same crime 500,000 times at once.