S&W seems to have worked out ALL the bugs in the M&P... save one.
Some M&Ps, mostly the 5" (like my Pro Series) and the 9L, are having some failure to extract issues, due to the additional slide weight, 16lb recoil springs (same spring as the 4" guns), and tight/rough chambers (due to internal melonite coatings).
The fix is simple. Polish the chamber, throat, and feed ramp (good idea for any gun), and if it is suffering from weak ejection of FTEs, swap out a 13lb recoil spring for the factory 16lb.
S&W is swapping out the barrels and springs on a good number of the 5" guns that are having issues. This is a very small percentage of the guns, but it's there. Very few 4"ers have ANY issues at all.
Low bore axis, good sights, and the gravely trigger is MUCH better after 400-500 rounds/dry-fires... Enjoy.