One thing to consider on piston uppers is the excellarated wear of the receiver extension and BCG. One design flaw of the piston system is the single contact point of the push rod on the BCG. This causes the BCG to push downward prior to moving to the rear causing the BCG to drag on the bottom of the receiver extension. This problem will not be eliminated until a double push rod system is developed.
The BCG tilt is one common complaint I have heard from some other knowledgeable people. It would be interesting to see how some of the various manufacturers try to address this. I'm also curious to see how HK tries to correct or minimize this in their MR series rifles.
Another related item that I have a question on is the various strokes of the piston I have seen mentioned. Some manufacturers specify theirs as a short stroke, while I thought some others stated long stroke. What is the benefit of either one?