I'd take a well-QC'd, well-made MIM part over a poorly-QC'd, poorly made forged or investment cast part. I think it has more to do with QC and how adept the manufacturer is at making the part than it does with whether it's MIM or forged/cast.
I believe both Knights Armament and LMT use MIM parts in actual military-issue firearms for folks who are actually using them in serious situations. I believe the MK27 adopted by SOCOM (Glock 19) also uses MIM parts. Certainly the M17/M18 uses MIM parts, and while those pistols have a troubled history, afaik none of it had directly to do with the parts being MIM (could be wrong).
That said, I can't deny that I'd take a well-QC'd, well-made forged or cast part over a well-QC'd, well-made MIM part. But I don't think the difference is enough to worry about.