The recoil spring in around the gas tube, on top of the barrel.  That's the primary weakness; the spring is subjected to great heat during rapid-fire, which wears out the spring.

Note that this is NOT a gas-piston upper as is commonly misunderstood; the gas system is slightly modified, but more or less works just like any other AR.  The main difference is that the recoil system has been relocated to the upper, allowing a folding stock.

Having had folding stock rifles, I personally don't like them.  I do like an ADJUSTABLE stock, which is what a standard AR stock allows, and yes, ZM makes a folding stock that is also adjustable (a nice but expensive feature).  Some folks really like folding stocks, though, and to each his own.

For myself, it's too proprietary and the configuration is limiting.  If you like it as-is, then you'll be happy with it.  None of my rifles are factory stock, though.  

-Troy