Ironically, in today's marketplace successful nonfiction has to be unbelievable, while successful fiction must be believable.
There is always a certain leap of faith that editors have made with their nonfiction writers. If the trust is broken, things can get very embarrassing for the writers and the publisher.
Writing nonfiction is more like sculpture, a matter of shaping the research into the finished thing. Novels are like paintings, specifically watercolors. Every stroke you put down you have to go with. Of course you can rewrite, but the original strokes are still there in the texture of the thing.
If uncovering the truth is the greatest challenge of nonfiction writing, it is also the greatest reward.
I still believe nonfiction is the most important literature to come out of the second half of the 20th century.