It just doesn't seem realistic that they would risk attacking the postman for no real profit.
Yeah, I agree. But you have to consider the source. Henry Hill has been discredited on so many things over the years, it's hard to keep track. The man is a pathological liar and a hopeless junkie.
I think that the real value of Goodfellas isn't so much that it's an account of Henry Hill's career (as you say, he's far from totally reliable), it's that in addition to being an extremely compelling and entertaining movie, it provides a very authentic and realistic portrayal of the larger Mafia world (at the level of many soldiers and associates, at least) during the time period it's set in. While this or that incident involving Hill may or may not have happened as depicted, everything that's shown
does greatly resemble things that happened in the real world.
