Reliability issues with cycling, chamber spec, a few other random things. Not all together, but enough gremlins to double check. PSA is, IMO, the middle man for manufacturers. Very rarely is an item "shipped same or next day"

My issue with the AR-10 platform.... lack of uniformity. You can't try your friends bcg at the range if it's not the same mfg. Because part A will not fit in gun b' s receiver.