The power thing is difficult to quantify though, MTB stuff is like an interval session, lots of explosive efforts, whilst road riding is generally much more steady state.
They achieve different things. If I go out for 4 hours on the MTB on my own I don't usually stop at all, maybe to top up drink. If I do a social ride it's much more stop start and slower. On the road in a group you can draft, and if you're riding with a group slower than you it's much easier than riding on your own.
The hardest ride I've done this year was at Afan early in the year, I was with 4 guys much quicker than me, I just got my legs ripped off for 5 hours! I've done 5 hour road rides that are easier than 2 hour MTB rides.
It's not the terrain, nor the type of bike that makes the ride hard/easy, it's how you approach it!