I was not trying to say that aviation’s autopilots are the same as motoring hands-free systems. Merely that both use computer data and calculations which can, and do, go wrong sometimes. Non-hands-free functions can easily go wrong: my Mini regularly turns on its “automatic” headlights during bright sunlight (dirty sensors?) and tells me, incorrectly, that tyre pressures have changed. The latter is caused by very hot or cold external temperatures. If such simple functions can be wrong then the more complex factors of autonomous driving can also go awry.
There is, however, one area where ground-vehicle factors are greatly more complex than aviation ones: conflicting traffic. While en-route an aircraft might have a few other flying machines which present a potential threat of collision. A ground vehicle will have many, many more potential threats: a minimum of 20 or 30 cars within range, then there are pedestrians, cyclists etc. The control guidance instructions needed for a car are infinitely more complex than for an air vehicle which spends most of its journey going in a relatively straight line and a constant speed with very little to bump into.