I'm not so sure. Especially with respect to the consumption of food.
In many cases, I can move an army into a wilderness or forest region with no city, and they can stay there, apparently using no supplies at all, for months on end. Their supply of food does not go down at all. Often, when I check the supply status of the regions (using the supply filter button) these regions are shown in red.
Are units consuming supplies like they are supposed to?