I would have to suspect the greatest weakness as well as the greatest strength of a GPS like system is ease-of-use
While its ease of use could be a godsend when you have other things that need your attention immediatly, it would be all too easy to become lazy about it
I mean, if the opperturnity was there, who would not want to just key in a few waypoints and then not have to realy think about where they are.. i mean, it tells you right there on the screen.
I guess, like any other instrument, it would require constant check and verify. they are useful tools, but getting lazy and assuming they are always right can kill .. and that is a discipline that requires work