Newps wrote:

Yeah and that's crucial condisering we're substituting for an ADF.

I guess that you mean that sarcastically. Without RAIM is is possible to have a sudden loss
of accuracy that would exceed the widths of an NDB final approach segment.

How likely? No more or less likely than having such a failure on a stand-alone RNAV IAP.

In any case, the criteria and conditions are set forth by Flight Standards, not me, not you,
and not ATC. ;-)