I believe that the problem with the concept of a flat tax is that it is only straightforward for salaried workers. What about self employed sorts? If you own a car repair shop and make $150000 in revenue but also spent $50000 on equipment and supplies how do you determine your income? Now you are right back to complex paperwork to figure all that out, which the flat tax was supposed to eliminate.

I am no expert on this nor do I wish to be.