This was an attitude I had for quite a while because of an offhand comment I heard from a guy at Knight’s Armament about 10 years ago. ‘It was designed as a impingement gun, we’re gonna keep building them as impingement guns’.
It might be more accurate to point out that the AR-15 isn’t precisely a direct-impingement gun. It uses an internally captured piston mechanism. Gas from the tube flows into the carrier key, expands in the piston chamber (near the gas rings), and pushes the bolt carrier backwards. The cam pin rotates the bolt while the carrier reverses, and once it unlocks the whole assembly moves directly rearwards as a unit to start extraction and ejection.
One of the larger issues with straight conversion kits is called ‘carrier tilt’. Long-stroke pistons use an operating rod between the bolt carrier and the gas port. These are attached directly to the bolt carrier, like the AK-47. Short-stroke systems use an op rod that ‘floats’ between the carrier and the gas port, and a piston inside the gas port assembly plays a game of Newton’s Cradle knocking back the op-rod and then the bolt carrier. Carrier tilt is much more noticeable with long-stroke piston designs. Short stroke systems vastly reduce carrier tilt but it can still be an issue with some conversion kits that don’t take this into account.
Piston-driven AR pattern rifles have been established as more than reliable. The HK416 has been in service since 2004 and is considered one of the most reliable piston ARs on the market.
So it’s not that a piston design is or isn’t inherently superior to the AR-15. The benefits of cleaner running and more efficient use of gas are very significant. But the DI system is much more economical to produce, because the piston design requires better engineering principles and production quality.
You’ll see people post-facto trying to defend their purchase on the basis of economics or anecdotal performance. As with everything gun related, the performance of your purchased firearm correlates to nothing so much as the quality of manufacturing work that goes into it.