What are the Advantages and Disadvantages of a Flat Tax?

-Supporters argue that it can Simplify the Tax System, therefore Reducing the Incentive to Evade and Avoid Payment and Increases the Incentive to Earn More -But this can bring Less Tax Overall and there is no Vertical Equity. It could be made Progressive if there was a Tax Free Allowance - Where no Tax is Paid until a Certain Income is Reached