Okay I won't say anything specifically on the Bushmaster/POF or other piston assemblies for the AR, but generally speaking, to assume that blowing all that crud back into the receiver does not have some derogatory effect on reliability is asinine. You can only drink so much Kool Aid, people.
The DI system has proven good enough for government work, as they say, but it makes the rifle a dirty, nasty bi--um, "female canine." The AR's DI gas system is getting no love in the immitation department. If it worked as well as AR Kool Aid drinkers thought it did, there would be other designs on the market immitating it. Alas, there is not. Piston driven systems, in particular short-piston assemblies, however, are the current rage, and reliability testing is confirming that, whether or not you want to admit it, this is for good reason.
Whether or not a piston driven rifle is generally more reliable than a DI rifle isn't up for debate. It is, for all intents and purposes, verifiable fact. DI will not be present on the rifle that replaces the M16, and once that happens, it will likely go the way of the do-do. The only thing left to debate is how much it really matters, esp given the demands of most civilian users. But even disregarding any increase in reliability, I think anything that makes the rifle run cooler and easier to clean bears consideration on its own.
That said, I'd look into a SIG556 before I considered an AR with a op-rod conversion. The SIG system is older, more mature, and proven.
a SIG of my own might be nice tho' seeing how Bushmaster seems to be dragging their heals with the Masada.
