When you load an M&P with a full mag you will notice that the slide will "auto forward" all on its own when slammed in with any significant force. Lots of folks experienced this with M&P's.

I dont use it normally since I was always taught to use the "over hand" or "over the top" technique. I'd only use the slide stop if I were doing a one handed mags change. But if I were forced to do a one hand mag change, my adrenaline will probably be off the charts and I wouldnt even notice a difficult slide stop lever?