I suggest that whether or not flying in varifocals is OK depends very much on what you fly. I wear varifocals normally, with distance vision lenses in the upper section fading to no prescription at the bottom for close vision. I also keep two pairs of standard glasses for flying, one clear one reactolite. I only ever once flew with the varifocals on, simply through forgetfulness. Never again!
The problem, at least for me, was to do with flying an open aircraft with a very wide visual field, both to the side and down. When helmeted and well-wrapped up against the cold, one's head movement is slightly restricted, with the result that your eyes are pretty much forced to move to the "wrong" bit of the lens. This was particularly bad when looking sideways into a turn, where the helmet tends to limit head movement a bit.
I can well imagine that flying in an enclosed cockpit with a relatively high glareshield would be perfectly OK in varifocals though, so it's almost certainly just an issue for some types of aircraft.
VP