I can understand the reasoning behind learn one firearm (or a family of firearms such as the M&P) and practice with it enough to gain through repetitive practice so that you point and aim the firearm in a consistent manner, operate the controls without much if any conscious thought such as the slide lock, and are fully aware of potential malfunctions and a remedy for those. Those who carry in public have to be even more careful.
For those that practice enough to establish unconscious mastery of the controls of various firearms, then carrying other firearms, assuming all other variables such as location of the draw etc. are similar, should not be a big a deal. However, I suspect that most folks, like me, don't have the time to master a bunch of different firearms and that simpler is better. I have used a revolver long enough to trust my life with one but also have moved with the times to a semi-auto that mimics the revolver with simple controls and a similar trigger pull (3rd gen DAO S&W). Evaluating the new (to me anyway--I stay about ten years back from trends) S&W M&P simply because of arthritis is making long heavy trigger pulls more difficult.