They were a little more reliable in times past, but you knew that already right?
From the time J. Edgar Hoover birthed the baby, the FBI has been heavily invested in self-promotion. I’d like to believe that the FBI has solved many difficult crimes, and that for most things, the FBI has been “reliable”, but have no way of knowing for sure. There are certainly many known cases of unreliability, including the failure to discern or catch extensive Soviet spy operations from the 1920s through the McCarty era, the FBI crime lab debacle, etc. A self-made myth of your own super-competence and incorruptibility sets the FBI up for looking worse than it probably is.