Fortunately, I haven't needed to return to my dealer since the 600-mile servicing for my 2014 S from Mission Motorcycles in Daly City. I found them to have a good staff and they were very knowledgeable about Zero motorcycles. Unfortunately, they went out of business last year when the shop owner got fed up with the whole vehicle-selling bureaucracy, retired and sold the building to The Dollar Store. However their Zero sales lady and the shop's Zero technician relocated to SF Moto, which seems like a nice store. But parking and traveling around San Francisco sucks, so I try to stay away from that shop unless I really needed their services - which I haven't so far, other than ordering a spare drive belt. My limited experience with SF Moto makes me think that the staff is competent, so I can recommend them if you don't mind traveling to SF and can find a place to park.
My only complaint about SF Moto is that they seem to have a huge backlog of motorcycles that are being serviced or repaired and I would be concerned about how long it would take them to get to your bike if it needed such service. Attached is a photo showing the alley where they park the bikes that are being stored during daylight until they can get around to working on them some day. I assume that they stuff them inside in the evening. I note that they apparently pay a homeless person to watch the bikes all day long so that they won't be stolen. He sits in a chair on the opposite side of the alley watching over the bikes that are waiting for parts to arrive or service to be scheduled.
The other other Zero dealer that I know of in the Bay Area is San Jose BMW/Zero/etc. But I have no experience with that shop.