The European Central Bank's (ECB) monetary policy "bazooka" have been the subject of some strong criticism and, even though some policymakers have sought to defend the bank, the head of Europe Sovereign Risk at Moody's told CNBC that the ECB's policies weren't as effective as they used to be.

"The ECB plays an important role in the euro area situation," Dietmar Hornung told CNBC on Monday, "but what we are now seeing is that monetary policy, instruments and measures are running into diminishing returns to those activities and that's something to be acknowledged."

"In terms of our ratings, the fiscal space compared to pre-crisis has been eroded to some extent and probably on the monetary side we see similar things and that's why we have our ratings in the euro area still pretty much at the same (position) that we had them in summer 2012 – because we acknowledge that the imminent crisis is over for quite some time but that structural fragilities remain."

Hornung's comments come at a difficult time for the ECB. The central bank is trying to stimulate growth in the 19-country euro zone but the region remains stuck in deflation territory and unemployment remains a mixed picture in the region.

Read More Euro zone data deluge: Better-than-expected growth, but we're back in deflation

Last March, the ECB launched its own 1 trillion euro ($1.14 trillion) quantitative easing (QE) program and in March this year, upped the ante by cutting its main interest rates further and expanding its massive bond-buying program to 80 billion euros ($90.3 billion) a month.

Ratings agency Moody's said in March that the euro zone sovereigns' ratings will likely remain stable in 2016-2017 "fading fiscal consolidation, limited progress on structural reforms and rising political risks limit upside potential and create longer-term risks."

The ECB's monetary policies are not universally popular – especially with German policymakers who are particularly critical of the bank's very low interest rate policies which hurt savers.

ECB President Mario Draghi has defended the bank and, this weekend, ECB policymaker Benoit Coeure hit back at critics, defending the bank's policies in a column in Germany's Frankfurter Allgemeine Sonntagszeitung newspaper, Reuters reported.

The central bank's executive board member said Germany would lose out if the ECB's inflation target or around, but just below, 2 percent was abandoned. He added that low interest rates and repeated stimulus continued to be critical to supporting employment and activity in euro zone economies.