The technique is still valuable. I.E. If you frequent an area plagued
by occasional fog, RDF observations recorded during periods of good
visibility would be useable when conditions were bad, even if GPS and
actual charts disagree.

Right on. RDF is most useful if the beacon is an actual marine navaid
on structures of interest, like an entrance jetty, which you can "home
in" on. Then, when fogbound and "close-in" , i.e., a few hundred
yards, it can tell you which side of the item you're passing on,
without benefit of a chart. It's less reliable when you're far
offshore or the signal emitter is not near shore (like a commercial
radio station antenna inland), because of the bending of the apparent
direction of the antenna by topography, etc.