I remember from my last M&P pistol armorer recert that there was a limited issue discovered with some "over-molded" extractors a couple of years ago (back at the time the LASD reported an issue with a small number of M&P 9's). That issue was identified and resolved. (The extractor is a MIM part, and S&W owns the MIM molds used, although the parts are made in a vendor MIM house, as I recall.)
Also, I can't remember when it occurred, but sometime about a couple years ago the engineers revised the M&P extractor to give it a bit more negative rake and a sharper point. Something about revising it to make it run better with some of the hotter loads some folks were using. (The same extractor is used in all calibers of the M&P pistol line, BTW, which is a neat engineering trick, and simplifies parts support.

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The MIM extractor is a robust design, with a beefed up front shoulder.
While we were told in my first M&P class that we should buy the .40/.357 Go/No-Go extractor bar gauge to check dimensions if we ever had to replace an extractor (in case it required fitting), in my last class we were told that current manufacturing tolerances made the extractors considered drop-in parts, and that bar gauges weren't going to be needed by armorers.
As a matter of fact, last time I asked, they haven't even considered making armorer bar gauges for the 9/.45 guns, as they feel they aren't needed if an extractor has to be replaced "in the field" (meaning outside the factory, by an agency armorer).
I bought a .40/.357 bar gauge after my first class, and I kept asking about when a 9 & .45 bar gauge would be available, simply because I like to own the tools as an armorer. (I have 9/.40/.45/10 gauges for the 3rd gen guns, and one for the standard SW1911 .45, too.) Every time I've asked, they keep telling me the engineers don't feel it's necessary due to the current manufacturing methods and tolerances.
They might be right, as I actually found an extractor that dropped into a 5906TSW for my last 3rd gen armorer recert a couple years ago, without having to be fit (filed).
First time I'd seen that happen in 4 armorer classes for the 3rd gen guns. They typically require fitting in any particular slide (and the extractors are made with extra metal on the adjustment pad, to allow for fitting in each slide). The instructor told me that he'd seen that happen in a couple of classes recently (at that time), and attributed it to the tight production tolerances and newer parts being made for the 3rd gen guns.
I haven't personally looked at, or used, the aftermarket extractor being discussed, but that's because as an armorer I use factory parts on guns intended for dedicated use as defensive weapons. (There's a pretty significant potential liability on me if I use a part not approved by the factory in a duty or off-duty pistol, and a problem occurs which is determined to involve non-factory part.)
As I recall when I looked at the part info, though, it's described as a
nearly drop-in part.
One thing that can be said for the aftermarket parts demand for the M&P pistol is that it has apparently driven S&W to continue to revise and refine their own parts.
