We know that it's geofenced, and I'm guessing anything within that is fair play. I don't see why time of day would be an issue, the 1 million trips probably come from the fact that they work all around the clock. Right now it's only available in Arizona, and weather is never an issue there. I also don't see why the geofenced area would contain regions with no data service.

In the New York Times interview a few weeks back, the CEO brought up the specific issue of store parking lots and loading groceries while the car waits, so it seems like they've thought of most corner cases.