I wonder if any of this has to do with wrist flexibility? With fingers facing forward, it might be that some find straight arm rolls tough on the wrists if they don't have adequate wrist hyperextension flexibility. I would think that most would, however (since the heel of the palm comes off the floor and the stretch is transferred into the fingers as the body hits the candle position); and that it is mostly a matter of being able to maintain tension strength/pressure against the floor as one's body is falling forward.
I know I have to either turn my hands in or out to do the handstand forward roll. I have the bad habit of turning them out due to lack of shoulder flexibility to go along with my lack of wrist flexation.