Don't the Springfield GI series use MIM extractors? Along with a bunch of other MIM parts, including the roll pins? I don't see heaps of them lying around in shattered pieces anywhere. Smith and Wesson does too (hammers at least, on some models), as do many gun makes.
My opinion - a lot of guns have been made in the past 20 or so years with lots of MIM parts. By and large, they seem to stand up just fine, or at least if they are experiencing massive numbers of failures, it's being well covered up. I know I don't loose any sleep over their presence (or absence for that matter).
Seems to me that MIM parts, if they have a problem, will suffer from fractures due to voids left after the molding process. But that is a QC issue, and applies equally to investment cast parts (eg. frames). Even forged parts can fail from hidden internal flaws in the stock steel used. As long as the QC is good, any and all of these parts should function just fine.