I think freestyle and trials stuff are probably a lot harder to learn on a bike, because on a bike you have to be much more careful to avoid serious injuries. Accidental falls off a unicycle usually leave you on your feet, whereas falls off or with a bike often get you hurt.
Yes. And, as I mentioned before, the outcome would depend on the course. For example, I doubt that any trials biker could do the line Ryan Atkins did (where there were 4 4x4’s in a tight square and he climbed from one to the next). More commonly, I think the average off-road unicyclist is better at trials than the average cross-country mountain biker. Again, it depends on terrain.
Still, you are describing how a person behaves.
In other words . . . what they DO.