Beside dead-end routing, GPS can give you very bad advise.
I was visiting friend in West Virginia and the route to Chicago wanted me to make 50 miles freeway loop on 250 miles distance. I check Google maps and it did the same. Maps were showing some small roads, that would save the 50 miles, but you know how small roads on the map can end.
So I ask my local friend and he said, there is very nice highway with good marking on the shorter route. I drove car at the time and the highway was so good that I was making freeway speeds anyway.
So all available mapping would give me running around.
