I own two holsters with this type of retention system. I used them to holster my M9 while I was over seas. One is a holster that will attach to webbing, and the other is a paddle hip holster.
I think that the model of gun your using, and the amount of practice you have with it will have the largest impact on how safe you are with that holster. For instance, the Beretta I was using has a slide mounted saftey that is not likely to be disengaged during the drawstroke. With practice, you learn how to release the retention switch and draw into a position that places your finger naturally along the slide/frame. However, model of firearm, and practice. Because of the practice I have with my Serpa(s) I would feel comfortable using them. I do feel comfortable using them. I carried an M9 in a Serpa on my chest for a year as a gunner during more conoys than i can remember. When downgraded into just ACUs I used my paddle Serpa.
I understand why its being banned from so many training institutes though. You cant trust every person who comes in with a Serpa to have enough practice with it, and you cant expect them to have a firearm with as inconvient a saftey as the Beretta. It takes twice the deliberate motion tosqipe off the Beretta's saftey than it does a 1911. The other major factor here, is that Tex Grebner changed his holsters. Practice makes permenent. Changing holsters doesnt fix muscle memory. Stick with what you practice.
Intelligence is something you expct from a gunowner. Especially someone who wants to attend a training class. BUT, we have to face the fact that some people are just not bright. Trainers have to take that into account when they look at those who attend their classes. Ergo, if you assume everyone is smart enough, practiced enough, and has a pistol that the Serpa works well for... your bound to be wrong somewhere. And that could lead you to an expensive (if not deadly) problem as a trainer.