I hate to be a claimer...  damnit..
But here's my opinion:
             Skiing- Backflips are the easier on kickers,  harder on cliffs.  Front flips are tough on a kicker, super easy on a cliff.
            Boards- Front flips are easier, and backflips are harder to land.
           Telemark: Front Flips are easier period,  and backies are just as hard as alpines.