Shimano used to specify front/rear just to ensure the brake pads were int he right orientation at the time of purchase, but if you swapped the pads left to right the rear are identical to the front.
THere also used to be Scott 'Self Energizing' brakes that were (as I recall) likely to grab uncontrollably and throw you over your bars if used on the front, but these have been off the market for a decade, I think.