Having ridden both and I would never put a road bike onto anything other than a nice smooth flat-pothole less road
That's why when I was out with my mates on their MTBs and they wanted a 3 mile offroad to show that they could beat me somewhere on the ride I took it steady on the downhills so I did not wreck the wheels and looked for the dry mud so I didn't get wheel spin on the uphills.
I used to ride the MTB on century rides with only a change to narrow slicks and a flipping of the stem. Boy was that hard work in comparison to the road bike. But on the safety side I don't think there is any difference between an MTB and a road bike when being ridden on Road surfaces. Hard pack and the road bike is slower and not as comfy but not dangerous in any way unless your energy exceeds your capabilities.