In its gun application infancy, MIM got a bad rap. Those bugs have been ironed out. Its quality depends on the vendor and how closely the process is monitored.
A few years back, a friend of mine wanted me to replace the MIM sear and disconnect in his 1991A1 "Billboard Rollmark" Colt despite my advice that he was wasting money. To no avail. He'd read it on the internet, you see...
When I got the swap finished, I arranged a scientific demonstration for him.
I laid the sear on an anvil...concave side down...and whacked it two or three times with a 4-ounce hammer. Not only did it not shatter into a thousand tiny pieces...when I installed it in a pistol, it functioned just fine, albeit with a rough trigger action because of the light damage done to the backside of the sear crown.
Then, I clamped the disconnect in a vise and whacked it. It bent about 15 degrees before it broke.
The biggest problem with MIM used in guns is that the manufacturers use it in applications that it's not well suited for. Assuming good MIM, it excells with frictional stresses, often...if not usually...outlasting machined steel counterparts. Under impact stresses, it doesn't tend to fare as well. Hammers offer an example. Nor does it tend to live long and prosper under shearing stresses unless the stresses are low and the part heavy in the cross section. Slidestop crosspins seem to do okay. Ejector pins, not so much.
As a rule, if an MIM part is going to fail, it usually does it early on. If the part survives for 500 cycles, it'll likely last for 50,000.
One thing I know for sure. Love it or hate it...it's here to stay. The good news is that, due to advancing technology, it will only get better and before much longer, nobody will want any of that old, obsolete machined barstock stuff that our grandfathers were stuck with.