The European Single Supervisory Mechanism can apply a consistent and strict standard of supervision and compare banks across national borders, meaning that risks to financial stability can be identified earlier on.
While it is true that it is primarily the nation states which decide on the use of macroprudential instruments, the European Central Bank, too, has been given macroprudential powers, and it can tighten national regulations. That makes sense and averts the danger of not addressing risks to financial system stability in good time.