Critics (including some opponents of the ACA who nevertheless support the concept of insurance exchanges) are concerned that the exchanges may become overly bureaucratic and impose excessive regulatory or benefit requirements that will restrict consumer choice and drive up costs. They also point to the enormous discretion that the federal government reserves to certify the exchanges, potentially restricting state options in design and implementation. These are legitimate concerns. Overly restrictive exchanges may fail to attract enough insurers to offer consumers and small businesses a wide variety of affordable plans that meet their needs. If an exchange offers only a handful of very expensive options, it may fall short of signing up a critical mass of healthy enrollees, leaving it with a population of very sick (and thus very expensive) subscribers. Over time, that imbalance could make the exchange financially unsustainable.