I would bet that the vast majority of M&P 10s work fine, sort of like how the vast majority of Glocks will never go kaboom, but a relatively few examples on the internet have convinced lots of people that it's a common occurrence.
That analogy with the Glocks doesn't work for the simple reason that there are - what? - a million Glock models floating around and, relative to that volume, only a small percentage of them (most typically, one of the "Fortay" models) have KABOOM-ed.
The M&P 308s, like the Ruger's piston 7.62 rifle mentioned above, are Johnny-come-latelys to the .308 AR market compared to the established players, like Armalite or DPMS. Colt's 901 is getting out there as well. Others include LaRue's OBRs. Price-points vary. But with the newer models there are fewer user experiences to assess, and the early reports of undergassing aren't encouraging. Not trashing the M&P at all. I really liked the .308 camo "hunting" model w/ the thicker barrel. Plus, from a business perspective, the 308 AR-market is too lucrative for Smith not to get the gas-port issue resolved, not to mention their own administrative interest in avoiding the headaches associated with rifles being returned for warranty work. (You shouldn't have to throw money at an AR 'smith to get your .308 gas-rifle running properly - that's the factory's job under warranty).
I've not had any problems with my M&P, but I have read about a some instances of short stroking, of those many seemed to get better with decent ammo and some break in. A few required the gas port to be opened up, but even that is not really the end of the world.
I agree the gas-port issue is resolvable, but with any of these .308/7.62 ARs you have to go into them with eyes wide open - in other words, you need to know up front
exactly what you're getting.
For example, there's the expensive, high-end GAP-10 .308 rifle. It's designed from the git as a "precision" gas-gun that'll shoot bug-holes out to any distance the shooter can see and hold. However, GAP's owner tells his customers up front that the GAP-10 is tightly-chambered and will reliably shoot
only .308 factory Match-grade ammo, typically within a small window of 168gn-178gn bullets loaded to a specific velocity. No 7.62 ammo at all. If you handload, there's no room for slop and he advises using
small-base .308 Match dies to guarantee reliable functioning. If you're okay with these limitations and have the cash to invest and a penchant for the meticulous handloading of Match-quality ammo, the GAP-10 is a outstanding .308 gas-gun and will hold it's value over time.
In my case I wasn't looking for "bug-holes."
I wanted a rifle that would run reliably with either 308 or mil-surp 7.62 ammo as well as, occasionally, my reloads. I was looking for reasonable accuracy from a 16" rifle and wanted one with the SR-25-pattern lower so it would accept PMags and avoid the need to get spendy on proprietary mags. In fact, LMT ships the MWS & LM8 with one 20-rd PMag.
My LM8 has run 100% regardless of ammo type, and it shoots superbly. Just what I know ...