It's not in straight line speed that they Uni is at a disadvantage, unless it's really blowing. It's at the spinnaker launch at A mark and even worse at the snuffing. There's a whole lot to be done there, quickly, and only 1 extra hand (if one is holding the tiller) to get it all done. If anything goes wrong, (spin halyard snags, etc.) you are screwed.

Kind of like a one legged man in an butt kicking contest!