I bet it would be easier on a dirtbike. They generally hold their line/position in the air better than a BMX. A BMX bike is easy to kick away from you or off line and land sideways since it is so light. Plus you have to spot the pedals before you land, they move in the air as well. I know that the first guy who actually pulled a body varial in BMX took forever and a day to finally pull it, as in years. FMX didn't take as long to progress. Probably will be the same for the special flip.
I'm just speculating though. I'm a BMXer for almost 20 years, but am brand spankin' new to dirtbikes.