A flat tax simplifies one little bit of math, and doesn't really address loopholes and deductions. If you have an income tax you have to calculate "income". If I have a little store and buy gum for $1.00 and sell it for $1.10, it will not work well if that's counted as $1.10 of income. That $1.00 is a deduction. We can't eliminate them. It's a "loophole" when we think the deduction is not in the spirit of calculating income, but that's not an objective criteria.

A flat tax removes the benefit of shifting income to different entities to pursue lower marginal taxes, but I don't think that's the kind of abuse we're looking at here, nor is that abuse particularly scalable.