Both sides have their points and the weak link in the whole subject is the makers/subcontractors and their combination of QC and pushing the envelope too far in the name of cheap.
When done well, MIM parts are marvelous.
They *can* break and supposedly more frequently than forged/machined. I've seen enough forged, machined, cast, stamped and filed-to-shape metal parts break that I'm not even close to suspecting that MIM fails more frequently than any other variety of gun part.
I'm okay with them being use on generally lump-shaped parts but not long skinny rod-type things that take anything other than longitudinal stresses. So nail-type firing pins, okay, hammers okay, slide stops are pushing it in most shapes, and L-type firing pins and M1 Garand extractors I think I'd rather have either forged or cast and "bumped" but I don't think anyone makes them that way. Those last two are the best examples I have personally seen of conventional machined stuff breaking, so it's really probably more of an inherently troublesome design.
AR extractors are another I have no problem being MIM.
BUT, and this is a HUGE issue for me, the makers/subcontractors and gun assemblers absolutely MUST properly inspect, gage, and preferably finish-machine the parts! When a mid-priced (high priced for me and my circle) Kimber 1911 won't drop the magazine when the release is pushed all the way in because the shape is WRONG on the side opposite the magazine's notch, that's just plain shoddy and stupid.
Imagine looking down into the magwell from the top of the pistol at slide-lock. There's a nice curve at the front of the mag well to match the front curve of the magazine itself. Push the mag release in from left to right and the curve hidden under the thumb-button is not wide enough, to metal intrudes into the magwell. That was not really what I expected from an $1,100+ "carry" pistol.
Can't remember what other MIM parts on it had problems, but none of them were from breakage.