I have broken MIM parts on a S&W revolver TWICE.
I broke the hammer block (a MIM part) on a J-frame. S&W sent me another one which was also MIM. I broke that part too. S&W sent me another one, which was not MIM. So far, it has not broken. I now also keep a non-MIM hammer block as a spare.
FYI, the gun was purchased new. It has never been dropped, hit, etc. But the gun has been shot a lot (although that should not matter in theory).
I don't fear guns with MIM parts (althought maybe I should!). But I disagree with people who say MIM parts are fine or people who worry about them breaking are paranoid.
Joe Mamma