I've been turned away too many times from firetrails, that GPS wanted me to enter to take it seriously.
Than it is government that does mapping that GPS uses, so it takes about 8 years for new freeways to show on the routing.
I felt pretty stupid driving 210 freeway in So California who as I check was 6 years old and my recent GPS showing swamps and asking me to turn around.
So if I am planning to take back roads, what I actually consider adventure, I go on google maps and zoom on satellite pictures. Takes good internet and some time, but will also allow you to notice some stuff map will never show.