I don't know anything about the M&P's, but my Ruger KP345 comes with both an internal lock and a mag safety. The internal lock was usued only three times. I was only curious. Other than that, it stays unlocked, and the keys are in their bag in the box. It's pointless and useless to me.
The mag safety never really bothered me. When I go to the range, I never shoot it without a mag inside anyway. So it's no big deal. I did, however, have a problem when I cleaned the gun and re-assembled it. I pressed the trigger by accident (just a liiiiiiiiiiiiittle bit) while re-attaching the slide to the frame in final re-assembly. THe mag release engaged, and the gun would NOT work at all. Trigger could be pulled, but hammer never struck. Also, with EMPTY mag inserted, it would do the same, and the mag would not drop. The mag release would not work. There is a small piece that jutted out from the side that had to be re-inserted. The only way to do that was to insert mag (empty), pull trigger and keep it pulled while using other hand to press piece back in, and releasing trigger at about the same time. Then, and only then would the gun function properly again. Dang thing was a pain in the arse. That's the only tme it affected me, and I didn't like that at all. Hope it never happens again.
I have heard of the rear sights being cleaned to fix this problem, but don't know how its done. But I would never do a thing like that.