I've built 6 ARs on Mega lowers. All had excellent finishes and four were flawless in every way... but two were slightly out-of-spec (by "slightly" I mean nothing that couldn't be handled by a "shadetree gunsmith"). One set of threads on each of the two (for the receiver extension and the PG screw, respectively) were too tight as received to be used as-is. (I tried numerous receiver extensions and PG screws from different manufacturers on both the OOS and other receivers before working on the receiver threads themselves - trust me, it was the receivers, not the parts.)
All of that being said, I wouldn't hesitate to use them again to build a budget-minded AR. My 6 were early production models and, as I said, 4 were flawless and the other two needed only very minor work. I also have no doubt that they would have exchanged them if I'd asked... I just decided to handle it myself since it was so minor.