I'm aware how tough it is to ride one, but a team of unicyclists have it easier than a solo MTBer!
I think this is right. Even doing mayhem as a solo, I think I probably put in less effort than the top solo guys. It is hard to ride a unicycle in terms of skill, but in terms of effort, you have similar limits to a biker, except that you spin out on the flat, and you don't have a freewheel. So, you put in a bit more effort on the downhills (although with a brake, it is less than you'd think), uphills are roughly the same, but on the flats, you can put in more effort on a bike.
I know I've done tens of thousands of miles of unicycling, and I never got quite as fit as I have from road biking 100 miles a week.
Also, because there's a smaller pool of unicyclists to choose from, the fittest bikers are probably way fitter than the fittest unicyclists, and probably able to put in a lot more effort.
Joe