The plus side of the MR556 is that unlike other "Piston AR's" its Military counterpart (the H&K 416) has begun to see some military acceptance. As others have mentioned standardization between most piston AR's is non existent. In 10 years I imagine a lot of the lesser known and less popular brands/models of Piston AR's will be sidelined due to a lack of parts.
Plus side of the MR556 Many special ops groups around the globe are beginning to use it and the USMC has introduce a variant into service.
I am not an expert but I am guessing a lot of the parts are interchangeable. That means that in the long run; if you plan to have the gun 10 or 20 years from now parts might be a bit easier to come up with.
Look at the H&K G-3; it's been around for more than a half a century. Parts are mostly interchangeable with clones and are plentiful.
Then look at the AR-18/180. Cool gun back when it came out; now parts are tough to find and it is a novelty.