In the case of a loan to the banking system the banks need to be deemed healthy and operations need to be carried out in exchange for adequate collateral. In the case of purchases of securities, public or private, the underlying situation of the issuer has to be considered as healthy and the debt sustainable.
Not only the ECB, but all the central banks of developed countries have implemented measures geared to the difficulty of the situation. I would point to the extraordinary decision of the Swiss National Bank to set a minimum exchange rate for the Swiss franc vis-à-vis the euro, making massive euro purchases.