I have about average luck with Narn beams but I've actually never launched a mag-gun attack that has failed to inflict terrible damage. This goes for both first and second edition. I've never missed, never caused light damage, and never failed to score at least one critical hit with the shot.
Thus far, second edition has only had one game decided by a freak beam hit. It was a re-creation of the EAS Alexander joining up with Sheridan (2 Hyperions vs. 1 Omega). One of the aft beams on a Hyperion blew half the Omega's damage in one hit and caused two serious and several minor critical effects.
Besides that, my beam luck has never seemed overly good or bad.