In monetary union, the participating countries can no longer adjust the exchange rates against each other. This means that what were once major barriers for entrepreneurs and consumers have now been eliminated: the exchange-rate risk, conversion and transaction costs.
Conversely, it means that the weaker the supervision, the stronger 1/4  BIS central bankers' speeches  the banks. The truth is that strict supervision leads to strong banks. Institutions become more resilient when supervisors require them to hold adequate capital to cover their risks and insist on sufficient liquidity and better risk management. The US institutions received comprehensive state aid during the crisis.