I am not comfortable with the pistons. Lots of proprietary systems that you depend on the manufacturer as the only source of parts. The bolt carrier was designed to move in the upper with a center line push and that's why the BC rails are so narrow. You start pushing with the piston rod on the top and you change the entire operation and you still have the narrow rails.
Rifles that were designed from the beginning to be piston like the SCAR, SIG or the AK have a more robust design in the bolt carrier area. I heard that the HK 416 has a problem with BC tipping and wearing the tube.
Just my opinion, I have a hard time trusting a modified system. For me the midlength is the way to go.