I felt Picard's rage in FC was costly but beliveable. The Borg appearances in TNG between TBOBW and the feature film First Contact were largely and wisely ignroed for the sake of dramatic license for this particular story.
I thought it was understandable to see Picard so upset. Yes, he's a pretty cool-headed captain, but everyone cracks under pressure (remember the episode where he was tortured by the Cardassians?). So it is believable to think that Picard has kept all this pent-up rage and guilt over the Borg using him to attack the Federation, and in FC it finally comes out.