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.
You have no clue what the FBI found or didn't find you only know what was revealed. Bear in mind that some Presidents wield a little more power over their administration than others.
There are rarely absolute truths in politics there are merely accepted truths generally designed to protect someone like the Warren Commission Truth or the TWA 800 Truth all manufactured to protect someone. I have no doubt that the FBI is often ordered to produce untrue reports. During the Hoover reign I suspect he often blackmailed his Boss and resisted certain instructions but I never doubted his love of country, I couldn't say that the same of a bunch of the later Directors.
The sun will come up in the morning, I count on that but if it doesn't, what does it matter to me? Same is true of your opinion.