Difference between CB liquidity tool and buying bonds:
When providing liquidity a central bank is effectively acting as a pawnbrokers, just on a massive scale. The ECB allows banks to swap assets (like mortgages which are hard or impossible to sell quickly) for immediate cash. Admittedly, some of the assets the ECB now accepts are more varied than previously, but even if banks go bust, losses to the ECB and hence Eurozone taxpayers are backstopped by real assets and should be minimal if not zero.
This is an activity that is fully expected of central banks and back in 2008 the Bank of England provided £185 billion to UK banks for up to 4 years under its special liquidity scheme of which the banks have now repaid something like 80% of.
Compared to providing liquidity, buying government bonds is a completely different kettle of fish. If one of the countries in question does default then the ECB, and hence Eurozone taxpayers, end up taking the whole loss potentially running to hundred of billions. That no longer makes it purely a low risk act of monetary policy but a major act of political or fiscal policy.
The only thing that will be accomplished for certain by the ECB buying more Government bonds is transferring billions of euros in risk from the banks to Eurozone taxpayers which is that last thing most individuals want.