The shooter is always ultimately accountable for their actions. The holster doesn't ND the gun in to your leg on it's own. That being said, I would like to pose a question:
What if a car's parking brake was not a lever, but was instead a button on the dash, placed directly next to another commonly used button such as the A/C or Radio power button? Although a person could operate their other buttons in a perfectly safe manner, would that not still be a serious design flaw?
Suffice to say, some things are just a bad idea. A person can learn to use a Serpa just fine, and be safe with it, until they don't. At the end of the day, it is still a bad design. I'd rather a competent shooter not only learn to be safe, but also learn not to use poorly designed gear in the first place and save themselves a whole lot of trouble.