ADF/NDBs are affected by all sorts of things and the further away you are the less reliable. The distance that they would be expected to work reasonably reliably is published in the AIP.
Thunderstorm activity, hills coasts and night versus day will all influence accuracy.
However the most important issue is that you get a relative bearing so if you are aiming off drift that'll skew the reading relative to the GPS.
Example if you are trying to track 080 to an NDB and you need to deal with 10degrees drift you would be heading say 070 and the ADF would be pointing 10degrees right if you are staying on track. The GPS will report 080 as track.
It's interesting from an academic point of view but ADF's are not a very useful Navaid unless you've got nothing else! I only use mine for IR training!