If the FMS says you are a mile off track but the ADF is on track which is right? Who knows? Our FMS is BRNAV so it must be within 5nm 95% of the time. That could be worse than the ADF.
Do you know what your FMS uses for the fix-ups?
It will be DME/DME or GPS. That will answer your question
BRNAV is just the approval category. It barely relates to the actual technology. I have a BRNAV (RNP5 equiv.) approved GPS but the only way I would be 5nm off track is if Ronald Reagan came back and turned the system off. With a paperwork exercise I can make it RNP0.3 which is probably closer to the actual accuracy
But surely if your FMS and your ADF disagreed massively, you would divert to some place with an ILS. I would.