The South had an unfair benefit with its complete emphasis on slavery.
At the time of the Constitutional convention, the vast majority of states were slave states. All states agreed to these rules and if states wanted to throw away the 3/5ths representation that slavery gave them, they had a right to do so.
But why would you call the benefit "unfair" if they all agreed to it?
The debate at the time reveals that had they not agreed to it, there would have been no Union. It might have saved everyone a lot of trouble in the long run for the slave states to remain in their own separate country.
But why would you call the supposed import tariff and its use “unfair” if they all agreed to it?
That is why a legal framework was to be followed. Because all agreed to make the status quo what it was and became.