RadAlt Distrust
Just a point for historical reference.
When I was flying off the boat in the fabulous Phantom II a long time ago, we had an SOP of requiring an operational RA for night carrier landings. This was a no-go item per SOP.
Radar altimeters of that day worked well when they worked, but had a much higher failure rate than present day ones for the same reasons listed in CONFiture's post #128. About 1/5 of my flights had an inoperative RA on post-start checks. What most of us did was go flying and gripe it on return. At least the RA was not integrated into the rest of the aircraft systems.
With that operational experience behind me, I am extremely distrustful of RA equipment and am amazed that present day system integrators have given RA equipment a position of high trust with so few possibilities for reconfiguration following an obvious failure.