Google something like "top causes of automobile accidents". No list I found had exogenous causes (rain, snow, etc) in the top 5. For machine driving to top that isn't going to be that hard given that we are our own worst enemy on the road.
There just aren't reliable statistics on what the safety of this tech will be at this point in time. All we have is extrapolations with lots of bias and little independent review. Maybe it will eventually be worth it, but that experiment will come with a great many human deaths and we can't simply brush aside the ethics of that. I personally think it won't prove to be safer than humans for many decades. This is far more of a complex problem than a lot of the developers think, and I believe there's a case of engineering arrogance going on here as well.
On the surface it seems like we have something very close to a viable product, but the remaining little details and issues are important and represent an enormous amount of work, where we are going to see diminishing returns and enormous difficulty making a system that is flexible and robust enough to handle all the required situations more reliably than the average driver on its own for thousands of miles, scaled up to millions of units around the world. They will be good enough for niche applications, but the dream of a general self-driving infrastructure is pretty far off.
You don’t need to be better than humans for all situations. You need to be safe. Humans are generally well above that threshold, except for when they’re drunk, high, tired, distracted, or otherwise stupid. It doesn’t really make sense to compare deaths per mile for cars vs attentive humans in stable conditions. Both can honestly be reduced to the point at which they’re practically 0 unless it’s the victim’s fault. We do need to be attentive to edge cases that cause cars not to be safe, but we can also observe the overall pattern and draw conclusions on how likely unknown edge cases are to be encountered.