The EU has shown that if you have a monetary union, without a political union, problems are bound to crop up.
Countries like Greece which could have benefited from fiscal stimulus and depreciation of currency don't have the room to implement it. So countries will be wary.
It is not problem of political vs economic union. The problem lies in different level of development of EU countries.
EU is not monolith. There are rich countries like Germany, France, UK and very poor like Greece, Poland, Estonia.
This will always be the case. In any country, you will have rich and poor regions.
If you have a political union, there can be a balancing act in terms of spending. Now you have some migration within the EU, but the poor countries continue to remain poor. Plus, individual country heads can cause a lot of trouble for the Union as a whole, because they are answerable only to their citizens.