Re your limitations question there are many things to consider. Clearly the low vis/minima autolands require incredible safeguards. Terrain on approach can limit Cat limitations as some airports simply not suitable for approaches less than Cat1.
Whilst all things in the flightdeck/on the aircraft need to have redundancy and be operational, the airport itself must also be able to provide accuracy, backup systems and safety for the operators. So things like Cat2/3 holding points, lighting, increased a/c spacing on approach, and back up electrical systems on the ground at the airport need to be guaranteed as well.
The list goes on.....but there are a few points.