I own two Serpas, know several fellow officers who use them as duty rigs, and none of us have had NDs. The design is okay, but there can be issues.
As Shawn Thompson stated, if there is bind, or excessive pressure, the booger hook can indeed slide into the trigger guard. After reading about this type of situation, I had to see if it could truly happen, as I was definitely on the bus that said, "no way!! The design places the finger at a slight upward angle upon draw and you have to move the finger into the trigger guard."
So, I unloaded my G23, made sure it was empty and proceeded to do what one would call speed draws. On more then one occasion, my finger did indeed slide into the trigger guard, although it never pulled the trigger. And the only thing I was doing differently was attempting to draw as fast as I could,and applying quite a bit more pressure then needed to actuate the release, in an effort to replicate a high stress scene.
I initially qualified with a Serpa, and had no issues with it, even though I was going as fast as I could. But then again, qualifying isn't what I would call high stress either.
In any case, I agree in this case it's mostly shooter related, as he himself stated. Going from different guns and gear, and attempting to stay super fast, and what appeared to be a slight hangup (in the slow-moe, you can see his pants ride up. with a proper Serpa draw, that doesn't happen IME) with the holster, lead to the ND.
BTW, for the Serpa owners out there, I suggest doing what I did. Unload that EDC, make sure it's safe, and practice with your Serpa going as silly fast as you can. While it might not happen much, I'll wager your finger will indeed slide into the trigger guard. But give it an honest try, try to be super fast as though your life depended on it. Again, like in my case, maybe you won't actually pull the trigger, but I think you'll get at least one attempt that will slide into the trigger guard.
Also, just because it can happen, doesn't mean it will. Just as because it hasn't happened doesn't mean it won't. The best safety is between our ears.
I still own and carry my Serpa holsters. I like them, and don't see myself getting rid of them anytime soon. I'm just more aware about training with them, and try to train as much as possible, with all my holsters.