The big problem with gps and backroads is a lot of back (dirt) roads aren't actually roads at all, just trails carved in over time by atv's horses, dirtbikes, jeeps, old logging roads, power lines, pipe lines, etc etc etc. So there wouldn't be any reason a gps company would include them on their software as they don't even have a name. So whether it gets reception or not, it's info is likely inaccurate. I do a lot of exploring on old logging roads in Maine and when my GPS is running, it thinks im in a large field. No road outlines in sight.
Edit: I should add my hiking gps unit knows a lot more of the logging roads than my car gps does. I use all of them.....car gps, handheld gps, google earth, and good old paper will never let you down...map books, atlas's, and the best if you're really out there topographic maps for the area you want to explore are worth their weight in gold.