I find the dual RADALT setup in our B212´s is pretty reliable up to the limit of the scale at 2500´ in that they do not fluctuate! or behave erratically however they are usually about +100-200´ different from the pressure altimeter.
As far as I know they are not gimballed and will be somewhat inaccurate due to slant range expecially since the txnm/rec antenna is mounted on the tailboom.
However when landing on glacier it will usually show 30´ RADALT! Definitely not to be trusted in whiteout conditions.