One chief disadvantage: virtually all proponents of the flat tax have to lie about it.

Specifically, they have to lie about the tax rate. They want small, tidy numbers -- 10%, 15% -- but at that level, they'd also need to cut hundreds of billions of dollars in US government spending or embrace huge and permanent deficits. (And given that most supporters of flat taxes tend to be big-time deficit hawks... well.)

To be revenue-neutral, a flat tax would need to be around 30%, meaning that a single person who earns less than $191,650 would see their taxes go up under a revenue-neutral flat tax.

Which is to say: this is a means of getting you to pay for a huge tax cut for the wealthy. And it's called "fairness", so how can you possibly argue with it?