I think sometimes their testimony is only part of it. Some don't even have to testify. They might have criminality and go and buy some drugs, etc., and then the authorities or the police get the evidence other ways so that the people plead guilty, in which case the testimony and the credibility of the protected criminal aren't even an issue. Sometimes the testimony is only one part of the case and the prosecution gets the other evidence otherwise.
And maybe it makes sense sometimes to recruit people who have been involved in crime because they're the easiest to recruit. You go to someone who is charged with a serious offence, who might draw 10 years in prison, and say, “Let's make a deal. We'll arrange for you to get one year in prison, and meanwhile you go out and infiltrate these other places.” And the other criminals who have known this person, who is a criminal, deal with them, and they are probably the likeliest person.
There are often people in prison who are informants, because their cellmates tell them what they did and they become informants as well.
And often the police don't have to rely on the testimony of the individual, but the information is there in order to get the conviction.
I think I would rely on what the police say, that from the point of view of getting their man or their woman, maybe they are successful. But the problem is that they often leave carnage behind the protected witnesses.