A SDC needs to be better than humans on average to be a safer choice. It doesn't need to be perfect. Now, getting humans to acknowledge that and embrace that is the real problem.
Practically it needs to be perfect, because whoever makes the self-driving car will be liable every time it kills someone. And there's going to be millions of these things.
Particularly since most drivers think they're above average. A car which is just better than the average driver might be a tough sell for them.