First, he notes that the advantage in terms of cleanliness is diminished when shooting with a suppressor:

I only run the guns with suppressors for testing when I did my comparison, and with suppressors, direct-impingement and piston-operated were both very dirty, ’cause the blowback comes to the chamber, not the gas tube. … On a piston gun or gas-impingement, the case is being extracted while the suppressor is still under pressure. Now you have all the pressure in that suppressor exiting both out the front and the back.

Also, we should mention the poor choice of platform for the piston conversion on a round receiver bore as found on the M16/M4 system. All other piston type systems out there utilize a railed receiverdesign, like the M14, AK-47, M249, FAL and so on. The round receiver bore design used on the M4 is only acceptable for the standard op system. The carrier and bolt expand on axis with the bore under the normal gas impingement cycle, but on a pistongun , you run into off center impulse issues with carrier tilt and incorrectly designed carrier contact points. Some designs attempt to address the carrier tilt problem with over sized carrier tails and rollers. I do not believe the receiver extension should be used in this manor. I know many people are very happy with their piston weapons. This is not meant as a knock on the piston conversion systems out there, but as a philosophical dialogue focused the new physiological relationships applied to the M16/M4 platform through the introduction of an operating system which has traditionally been applied to receivers with rails for the bolt and/or carrier. I would rather see an entirely new weapon system designed for the piston from the ground up.

See my upcoming review of the XCR!

I should also note that POF-USA has long offered piston AR-15 uppers at premium prices, similar to LWRC.