I'm guessing that knowledge is from all your hands on experience?
I've seen these as kit guns,and full uppers,and both were fanstststic,the full uppers uses a standard gas system as opposed to blow back operation,and were awesome little guns,the kits were "ok",but not comparable to the full uppers.
Most issues I saw were mag related.more specifically mag block related.If someone made a dedicated lower,that issue could be solved easily.