RNP-AR
There are issues with terrain masking and RAIM outages, but you look at these prior to dispatch.
As has been pointed out, if the GPS goes you revert to standard drift rate of the IRS so not a major problem immediately.
We have a procedure of checking the altimeters at the FAF and they have to be within a certain tolerance (can't remember just now as we don't have approval just yet).
If you want a gross error check on vertical profile, I tend to use 320' per nautical mile for a 3 degree glide.........not too difficult.