Sagan, as he said himself, is used to a 5,6 or 7 hour face with only an hour maximum at high intensity. This is more at higher intensity. It's a different skill set physiologically, and he could struggle being on the anaerobic threshold the whole time.
It will be interesting. Watching the womens race, I could see women resting at times. In cyclo cross, it seems 110% effort at all times, cyclocross has been described to me to be similar to criterium racing. I have seen MTB WC, along with Swiss National champion racing side by side, at a summit, able to converse with one another and fans, racing MTB at sea otter in Monterey. Olympic race, I am sure at another level, the women, outside of the Swedish Gold medalist, and Polish silver medalists seemed to have ridden out of their skins. Since Sagan is starting in last place he will need to pass many non contenders, on a narrow technical course. Riders of a different caliber and skill level. As far as road riding, I would place Merckx at the top, he broke many records during his hour record. Sagan, in the Tour of California hung on to win against a superior climber Alliphillipe. Sagan, Merckx, DeVlaemink,(multiple World cyclocross champion) has the versatility to win gold Sunday. That is not a put down on all the Olympic MTB, contenders, I think that great champions such as Peter can rise to the occasion.