Correct. MIM in and of itself is not necessarily a problem. Porsche (and I think BMW or Mercedes) even uses MIM in certain engine components. MIM can be done in a quality manner. With HK, for example, you generally never see any issues from the components that are MIM.
With SIG, at least in the past, one issue is that they've apparently repeatedly switched between parts subcontractors at various times. That isn't an uncommon industry practice. At certain times, this seems to have led to a few issues with QC or metallurgy, parts breakages, out of spec parts, etc.
Most of my own limited experience seeing this first hand was about a decade ago working at multiple gun stores, a range, and on the wholesale side, so take from that what you will. By most accounts now, it seems these issues are considerably less common with SIG, specifically with the classic P-series.
In hindsight, I wish I could edit my poorly-worded original post so it doesn't sound like I'm implying that there's still a substantial problem with the classic P-series. My post was more in response to Sero Sed Serio's in which he raised the QC issue.
My response was more to point out that, if that was an issue with anyone's gun, there are now quality aftermarket options for nearly all of the components that they might have a problem with, so it's just not a big deal worth worrying about. Sorry for the confusion.