If they launch in one city or a few select cities, I don't think that signal tells you that much.

It is true that if their tech can't handle rain well, they would avoid rainy cities. But even if their tech can handle it pretty well, why pick a city that has less than maximum odds for success? Going for the low-hanging fruit first could very easily just mean that they know how to prioritize, not that they can't reach the higher fruit.

Also, there are lots of other factors (like regulation and other driving conditions) that dilute the signal.