There were many sceptics: It can’t happen, It’s a bad idea, and It can’t last, as Rudiger Dornbusch has nicely summed up the arguments of the critics (Jonung and Drea 2009). The intellectual tool at the heart of the discussion on monetary union was the theory of optimum currency areas (OCA), which goes back to Mundell (1961).
The argument is well known: Countries or regions facing asymmetric shocks need different monetary policies and an adjustment in the exchange rate.