With no national boundaries there is not importing or exporting within the EU, just like in the U.S.
No national central banks means when Italy imports BMWs from Germany, the debt that is owed to BMW doesn’t also become an Italy National bank debt. Inter-national trade in Europe ends up having to be settled by the national banks of respective countries. In the super state the debt will be owed only by the buyer of the car and owed only to BMW (or rather the lending institution). Inter-national trade becomes intra-national trade. Or inter-state trade, like in the U.S. There are no longer any national banks. Only one central bank.
Welfare becomes federalized. This theoretically benefits misfit nations like Greece and Italy and the burden falls on Germany and France. Had England no left the union it would fall on them too. This is just like in the U.S. Wealthy states pay more for the welfare in poorer states. It equalized the benefits.
Democracy is the problem. Will they have one man one vote? Then smaller previous nations will be at a disadvantage. Will they have an electoral college for president? That also puts smaller former nations at a disadvantage.
Just like in the U.S. ! I don’t think the U.S. would form today. I don’t think a United States of Europe can form today.