The nitpickers say that the retirees that have all of their savings in after tax money will be hurt. In a stretch that COULD be right for the wealthier ones. If so, it should be easy to do some Peter/Paul type of thing with the legislation since the pre-taxers will receive such a HUGE bump in their buying power.
If you'd do some rational examples using reasonable numbers for the tax free part of spending under the FairTax to get to the likely effective tax rate, I believe you'd find things much less alarming.