I've had GPS dropouts, admittedly always on portable rather than built in systems. The most severe I can recall lasted about 70nm from approximately Chester to approximately Northampton.
It does concentrate the mind somewhat. Nothing at-all wrong in my world view in using GPS as your primary navigational source. But using it as your sole navigational source will, sooner or later, land you in the doo-doo. Fortunately, I don't.
G