I used to be a fan of the Serpa holster.  However, the more and more I used them the more problems I had.   I don’t recommend them to anyone and suggest just about anything else over the Serpa.  A brown paper sack is preferable in my opinion.
Problems I’ve personally seen:
1.  Failure to depress lock mechanism completely, which results in the pistol not releasing during a quick draw.
2.  The gun being jerked out of the hand somehow, resulting in a draw stroke that ends with the gun being tossed away from the shooter.
3.  The lock failing and ending up with the gun being stuck in the holster.