Common to all monetary policy decisions is that their transmission to the real economy is sluggish and can sometimes be incomplete. For example, the extent to which banks are able to lower lending rates following a rate cut depends on many factors beyond the control of monetary policymakers.
One can think of the level of competition in the banking sector, or the health of bank balance sheets. A successful Capital Markets Union will open up entirely new channels for the transmission of monetary policy to the real economy, making central bankers much less dependent on a single sector for the transmission of its policy decisions.