One obvious consideration is that central banks, in managing their own balance sheets, need to assess and manage risk across a wider and much larger pool of assets. Gone are the comfortable days of holding a modest portfolio of bonds issued by the home government that were seen as of undoubted credit quality. Central bank portfolios today have more risk.
When there is not much room for countercyclical fiscal policy, it is expected that monetary policy should provide a boost to economic activity. However, its countercyclical potentials depend on the nature and strength of inflationary pressures.