It comes down to this:
Nothing is perfect.
The Stoner Gas System was designed to cure the many problems that are present in an overhead piston design, by moving the piston into the bolt carrier behind the bolt. And that's what it did.
This cured the piston problems, but there was an imperfection because there is carbon-laden gas going into the carrier. However, virtually ALL of the major problems present in overhead gas piston systems were CURED.
Since nothing is perfect, there is a choice here as to whether to return to the laundry-list of problems associated with an overhead piston system, or to accept the one small problem of the Stoner Gas System, which came along with the cure for overhead pistons.
Rarely will anyone argue against the statement that there is definitely an advantage to the Stoner System in the AR15, regarding accuracy and controllability. These are the strong points that came with moving the gas piston behind the bolt. The AR15 with Stoner System is the fastest shot-to-shot carbine available, and also the most accurate. In normal conditions, it has no peer in actual shooting performance as a combat carbine or rifle.
Most piston proponents will point to the "sand reliability test", where in the worst possible conditions for the weakness of the Stoner Gas System design, the AR15 fared about a half-percent more likely to have some form of stoppage, which is beyond the full ammo load of a soldier. Moot point, totally.
This is "all sizzle and no steak".
You MIGHT gain some infinitesimally small advantage in stoppages, IF you fire the gun without lubing for more than 300 rounds without stopping, in a sandblast cabinet, compared against the SCAR or latest XM8.
There is NO WAY that can be extrapolated to mean that any piston system that you slop into the gun will be any advantage. In some cases shown on some threads on this board, some piston conversions can't even make 1000 rounds before it mechanically breaks, which is ironic when you are buying it to get supposedly improved reliability.
And you will definitely lose accuracy and controllability in normal conditions, whether you want to admit it or not.
And you will pay a lot more money.
And you'll have unnecessary wear on other parts that people don't talk about, from stuff like carrier tilt.
Here's the facts. In almost every military trial on the globe over 40 years time, there has been nothing that could consistently beat the AR15 in an all-out head-to-head competition. And all the competitors have been overhead piston operated guns.
Even right up to this very moment, there is nothing that can beat an AR15 in all the necessary categories to win out. Nothing. Not the SCAR. Not the HK416. Not the XM8. Nothing.
You have to try to tilt the field in a direction which "weights" the one area of AR15 weakness(high round count sand-cabinet test), in order for ANYTHING else to beat it . And even then, when you do everything possible to put it at its most vulnerable disadvantage, you can only get a half-percent better IN ONE AREA, and THAT'S ALL. And it was CLEAR, that the major amount of stoppages for the AR15 were "stacked up" after everybody knew it was going to need lube after that round count, and it wasn't lubed, so there you go. If it was lubed on schedule, then it would have not even shown a disadvantage in that area.
Now, to be fair, everybody can buy what they want. It's a free country, and there are many nice guns out there.
It doesn't have to be "better than an AR" to be a very nice gun.
But, it seems that there are some who can't sit for that. They MUST find some reason that what "they want" is BETTER THAN AN AR. And that might be possible, but it hasn't happened yet. When it does, we'll all know about it.