From what I understand, most of it is plastic. But there's many different levels of plastic quality. Wondering if it's a cheap plastic or a durable one before I buy one and drop a geissele trigger in it and possibly SBR it.

a couple of friends have one, and they shoot it for a modified 3 gun and run and gun carbine matches... only problem they have commented about is that both have broken extractors

The polymer on the 15-22 does not like heat which can lead to the barrel nut coming loose.  A friend uses a slidefire on his and only issue he's add was broken firing pins.  S&W CS sent him a couple extra firing pins after the first one broke and it had several thousand rounds on it.  Just don't leave it sitting in the hot sun and you'll probably be OK.  I prefer my dedicated uppers because they feel more like my centerfire AR's.

I've had mine for 4 years or so and shot it in several steel challenge and other matches without any issue. I have a warlock II mounted and it also shoots fine suppressed or not. With a Weaver Classic 1-3x, this is the best squirrel rig I have had.

I have the Performance Center variant.  Mine has the threaded barrel which appears to be gone in the current product.  Great gun and I enjoy it often.  The local raccoon and groundhog population would disagree but at least their population is now under control.

I have shot thousands of rounds through my first rifle, then bought another this past winter with the M lock foreend.
Not a whisp of trouble in either. They both shoot very accurate and are easy to clean.
Its a pretty tough type of plastic.

They hold up real well.  Mine has 15000-20000 rounds through it and still going strong.  I did change out recoil spring for a new one at around 12k.  Btw currently running a Geissele 3gun trigger with no issues what so ever.  Have fun.

I keep my .22 ammo in .30 cal cans. They will hold 3500 rounds. I got my first MP in 2009. This coincided with my kids being of an age where they could start "run and gun" type shooting. That first rifle I KNOW for a fact has digested 8 cans of ammo. I've killed two extractors and a recoil spring. BUT, the rifle just keeps trucking on! Rarely have any kind of issue with it and when I do, it's ammo related 99.9% of the time. Mine has been very accurate as well. They're just great rifles....I've bought two more since and will never be without one. A first class training tool.