What Disco_Dante said.
In the GBBR world, "open bolt" is a misnomer: "closed bolt" refers to an older system where the bolt carrier cycles but gas system does not (you can see it when the bolt carrier moves back); "open bolt" refers to a more realistic system where the gas nozzle sits in the place of the bolt and moves with the bolt carrier, much as with the blowback unit in the slide of a GBB pistol.
As far as reliability, its important to keep in mind there are several generations of technology which was unleashed in very short succession (WE putting out 4 gens in about as many years). Newer versions are much more reliable, even out of the box; there's not much that NEEDS to be done except for the addition of an NPAS.
Of course, as with all things, certain parts are prone to premature wear, breaking etc. and a few models are particularly bad.
But unlike AEGs, just like the shooting experience being more realistic, expect to have to do more realistic maintenance on your GBBR -- albeit not nearly as time consuming or annoying as working on a mechbox. But I know some people just play with their AEGs for a year+ without doing more to it than charging the battery. GBBRs have greased parts, exposed to the elements to attract dust, sand and dirt, they only have a few moving parts but they move with considerable force and violence. You need to maintain your gun.