Indeed, there can be little doubt that the euro is a sound currency. The mandate of the European Central Bank to maintain a stable purchasing power of the currency is doubtless firmer than that of the Federal Reserve or any other major central bank.
And this is the first important reason why banking union matters for capital markets union: the banking union has been designed to increase the resilience of banks and a more resilient banking system supports the development of capital markets, which is one of the objectives of capital markets union.