There are three issues with the SERPA : 1) bad design contributing to NDs; 2) Durability; and 3) Snow, dirt or debris locking up the mechanism.
Dr. Glen Meyer, a Professor who is also a shooter gives a good explanation why the SERPA is a poor design from a Human factors standpoint:
"Given the human factors of finger usage that the problem is that people don't do the smooth flat release and tend to hook the finger to press the lever. This is a natural occurrence given the way we use fingers to press buttons and also continues the natural motion to get the finger to the trigger. Gun design is such that the finger seeks the trigger. This is called an affordance (see Donald Norman - Design of Everyday Things) and leads to errors. They are terribly hard to train out of. Sliding the finger in a controlled environment as a TV shoot might make it easy. Under stress, you press with the finger tip and that tip heads right for the trigger. It is hard enough to keep fingers off the trigger with regular holsters or other retention systems. Putting the finger into a tension filled motion seems pragmatic. Even with lots of reps, we know that folks can circumvent muscle memory. Muscle memory must be correctly recalled (not consciously though) to be used. It's retrieval can be over ridden in stress.
Thus, to repeat myself, the discussion of 4 rules is really irrelevant to the problem. Pilots learn that they should not stall but they do and crash when the stress, systems and evaluation get out of sync.
PS - in the video, you can see the flat finger start to curl in as I said was the tendency. In fact, I might argue that using the finger to apply pressure might enhance this motion as compared to a regular draw where you don't get your finger in motion until you are on target."