The FBI's goal is to arrest people, charge them, and send them to jail. Everything they do will be presented to the public, at trial, and examined by the opposing side and presented to a jury. There's not a lot of room for nuance here. There's also not a lot of room for secrecy. The accused has the right to confront their accuser.
The CIA's goal is to "protect the nation" and almost by definition deals entirely in secrets and ambiguity. "Sources and methods" are some of the most closely held and valuable secrets. "A person we kinda trust, sometimes, accused someone of possibly planning to maybe do something nefarious." -- not going to hold up well in court, even if it's totally reasonable to now put that person on some kind of watch list (and yes, at this point, all of us are on some sort of watch list -- see the disclaimer above; we're now at 30,000 days of the condor and counting)
So -- intelligence sharing between the CIA and the FBI sounds great in theory; in practice it may be tricky.