Mountain saddle on a road bike is, IMO, fine. Road saddle on a mountain bike can get sketchy depending on your weight and riding style. I say this because road saddles tend to be lighter and less durable than your standard run of the mill mountain bike saddle and may not hold up as well. A mountain saddle should hold up fine on a road bike, but may be a bit heavier (who cares if it's comfortable!).

That said I think my road bike saddles are more comfortable than my mountain bike saddle (same brand, same width) because they are less padded and flex more. Go figure.

Getting a mountain bike saddle with more thickness/padding may mean you need to adjust your shorts preference to better suit.