The old saw (Pareto) says 80 percent of the cases can be handled with 20 percent of the effort. An extrapolation would be: self-driving cars can handle 80 percent of the driving very easily but it's the last 5 percent, especially the last 1 percent of the time which is difficult. Which I find to be rather like the old "last mile" problem in cable: easy enough to move data across country in a flash, but getting it the last mile was difficult.
Nissan has an answer, whether it's workable remains to be seen. They're using a telecenter to handle the unexpected problems (like a emergency road crew patching potholes or something).