Tom Redburn nails it:
Whatever the rate, critics say, a steep federal retail tax, piled on
top of existing state sales taxes, would encourage widespread illegal
tax evasion, black market transactions and other forms of cheating,
creating a cycle that would require even higher tax rates.
“The main weakness of the FairTax is its comprehensiveness,” said Dale W. Jorgenson,
an economist at Harvard who opposes the plan but whose research into
problems with the current system is sometimes cited by supporters. “It
tries to roll everything into one tax, which simply can’t carry all
Response to comments: Note that a "fair tax," or a national sales tax, isn’t the same thing as a standard multi-stage VAT (a better idea); for a standard VAT the dual reporting requirements make it self-enforcing to a much higher degree.