From what I've seen, it is generally that people get the timing off when they are in a rush and try to push the safety down when they are already pulling the trigger. I doubt they are seriously unable to perform these actions under normal circumstances, but throw in the excitement of a competition or the stress of a real shooting and things don't go as planned....
It makes the case for avoiding systems that require sequential actions to fire.
Huh. I think any defensive use of a gun requires sequential actions, regardless of the controls of the firearm in question. You have to get a grip on the gun, then draw it from the holster (or other starting place), then point it at the target, then pull the trigger (or not, depending on what has happened to the perceived threat while you were doing the foregoing). If being under stress means sequential actions are impossible to manage, then the anti-gunners are right, and there's no point carrying a gun at all... you're just doomed to shoot yourself or some innocent bystander.
In reality, stress poses problems. But for someone who is a regular user of frame-mounted, down-to-fire safeties, stress doesn't interfere with that aspect of things. The safety comes off as part of the normal grip on the gun. No conscious thought is required. No remembering is required. No active sequencing is required - the safety is off long before the finger should even touch the trigger.