It's not about what the autopilot does, but what the driver/pilot has to do while it's on. In a plane, you don't need to be ready to take over instantly at any moment with autopilot on, right?
I think the pilot absolutely has to be ready to take over at any moment. I know that at least on general aviation aircraft the autopilot can automatically disconnect for any number of reasons, including various weather conditions. When that happens the pilot has to immediately resume hand-flying.
