I recently tried out the Coupons feature in my payment form, but ran into a limitation with it whereby the discount is applied to the Total Amount before the custom payment rules are applied. In our use case, we use custom payment rules to determine the total amount payable (e.g. calculating the price based on selections the user has made in the form). So for Coupons to be useful, we need it to apply the discount after the calculation rules.
Per my discussion with Thomas, they mean to allow percentage-based coupons after custom pricing rules.
Adding a vote to be able to have % coupons used in combination with custom pricing rules!