Over at the Washington Post, Ed O'Keefe voices some of the concerns. The biggest one is rather too subtle to ever make a good talking point, but I think it's important. "But giving OPM oversight of a new health-care plan sets it up for much more criticism and attention than it's accustomed to. OPM is a small but important agency with a limited focus and virtually no enforcement powers. The agency usually garners the attention of only a few lawmakers, the small but powerful federal workers unions and the inside-the-Beltway federal workers trade press (as well as a few national reporters, including yours truly)."
I have a theory that agencies like this work relatively well because all the relevant stakeholders are deeply involved in the process, and legislators are not going to do random things to them in order to please some constituency. The more opportunities your agency offers for political grandstanding, the worse run, on average, it will probably be. OPM is not perfect, but from everything I know about it, it does a pretty good job with a minimum of fuss.
But if you bring huge numbers of new people into its purview, that changes. They're going to get a lot more mandates--a lot more contradictory mandates, or mandates that impede the core mission--because suddenly, they're visible.