There have been numerous NDs with Serpa holsters because the locking mechanism forces the trigger finger to be too close to the trigger after the draw. The trigger finger should be on the slide not the bottom edge of the frame where the Serpa places it. Combine this with the requirement of the trigger finger to be pressing inward to unlock the gun leads to problems. There have also been problems with dirt and pebbles getting in the locking mechanism and locking up the gun in the holster.
Tactical Response, Suarez International (and two others that I know of but can't remember the name) also ban the Serpa