I've had problems with AREA 1000 pistons and Gaurder pistons. I'm aware that quality can vary from batch to batch and that different brands of gears can affect how long teeth on the piston will keep from stripping. There's a load of stuff that affects pistons...temperature, design, chemicals involved, etc.
Would I buy Gaurder or Systema pistons again, yes. Do I use them now, yes. Do I have a favorite piston that hasn't crapped out on me yet, yes. Will it eventually crap out, yes.
http://www.airsoftmechanics.com/reviews.php?aid=5
This test is not the be all, end all. It is simply what happened on those days of testing. For $20, I think there are better choices a person can make when buying a piston. In my experience, not all poly carb pistons are the same. There's always going to be a company charging twice as much for something that is only slightly better, or sometimes, slightly worse.