There have been self-driving trains for some time now (Nuremberg, London Docklands). There are more people to be responsible for on a train and I do not see why a bus would be a special case.
The London docklands railway have a staff member on board who opens/closes the doors and starts the train moving. It's a less skilled (and therefore almost certainly cheaper) job than driving a train, but it's still a person onboard.
Apparently not all the trains in Nuremberg are staffed, so perhaps it does work? I suppose the train driver is often segregated from the passengers in a way which makes them almost absent anyway.
It will be interesting to see if completely unstaffed buses are adopted anywhere, and if that proves succesful. I'd imagine that security wise a staff member isn't really necessary during busy day time routes (but then conversely that's probably when they'd be the most cost effective and provide the highest utility).
Not really. In case of a problem on board a train (unrelated to train's motion; for example, a medical emergency or a rowdy passenger), the train can usually be met by someone at the next station. In case of a bus, much more often there is no one to meet it at the next bus stop.
