MATRIX: A Murdered Journalist and the Mafia's Crypto-Phones. Cracking the app’s code and the investigation into organized crime in Athens.

How was that a failure?

The original guy that ordered the crime has still not been found, right?
Wasn't that their aim anyway? All the people caught, were probably just expendable and the leader couldn't get a full conviction.

So the result seems to match their operational policy.