I suspect that if rest is forbidden in some areas, they will be definitely rare. I doubt if they suddenly forbid resting in a location such as a goblin camp or an underdark there are far too many fights there (I can already imagine how annoying an underdark would be if you had to come back to the surface after each fight).
In my experience, it's definitely possible to clear the goblin camp in a single "day" without resting, if you pace yourself right. Logically I think it should be a "no resting" area. Can't speak for the Underdark.
