From what I've seen with Waymo, autonomous driving in relatively good conditions (light to medium rain, fog, or sunny) is a solved technical problem, even in small, winding streets. Snow is the next big hurdle, and they're actively working on that in Denver and Detroit.
They're being conservative with their rollout mostly because of civic issues. Most places do not have a legal framework for "what if your autonomous vehicle hits someone?" yet. Even if Waymos never were at fault for a collision with a person, you can always have cases like the one in Georgia back in October where a bicyclist wasn't looking where they were going and rammed into one. The shaky legal ground is a pretty big impediment right now, and that's in the US where we have much laxer laws about corporations killing people.