I don't think it is an instance of "No True Scotsman" to say that there are situations that self-driving cars may never be as good as humans at, but that those situations only occur because of the unpredictability of human drivers.

Self-driving cars don't have to be better than humans at everything in order to be objectively better drivers than humans. In 2010, 32,999 people died in car crashes in the US [0]. If self-driving cars are on-balance 1% better than human beings at driving, then 329 lives are saved.

[0]: https://en.wikipedia.org/wiki/List_of_motor_vehicle_deaths_i...