I think driving around the congested parts of San Francisco would be more difficult that driving on a mountain road (I have done both). Many more unpredictable events in the City; double-parked vehicles and clueless or agressive pedestrians, bicyclists, motorcyclists (lane splitting is legal here), and drivers, to name a few. Waymo seems to have done ok. Not perfect, but OK.
Not all mountain roads are created equal.
Human drivers are far from perfect.
All I'm asking for is comparative data that cover the full range of conditions that will be encountered.