Off the top of my head i think the feed lips on the mags are reputed to be easily knackered, but I believe that the main concern is that the whole design isn't reliable. This may be prejudice however because some SRC guns have a well deserved bad rep (M4's) and some people just don't like many of their guns because they often use proprietary parts which cannot be upgraded with aftermarket parts easily (I have one of their G36's and, apart from the hop rubber it comes with, it is good, but it does have a proprietary hop chamber which doesn't do anything differently to the TM design most use, it's just not quite the same shape, which means the best upgrade is the one it came with - SRC's own pot metal design as opposed to their plastic design, but it works fine anyway so no problem, except... if, as i did, you lose the little rod which pushes the rubber onto the BB in place of a standard nub, you cannot buy any old ACM hop unit just for that part, you have to get an SRC - this bugs me, as does the fact that the gun will never evolve - there is no CNC 7075 ali hop upgrade for eg). You may find that there are similar differences in their GBBR design.
I'm not into GBB at all though - even my pistols are NBB, so I tend to skim/skip posts in forums/fb which concern them.
