"Accident waiting to happen when you add in a pistol like a glock or M&P "
What exactly are you trying to say? What makes them more dangerous for appendix carry than other brands?
The trigger is the safety on both of those. There's substantially less forgiveness in regards to the trigger with these than say, a DA pistol or something with external safeties.
He could be insinuating that there's an increased risk of snagging something inside the trigger guard while repeatedly drawing from appendix. That'd be my guess. One less thing for him and other instructors to worry about when dealing with students of varying skill levels.