A setback for writing a "non-fiction-fiction novel" would be perhaps that the reader is found having to conform to a timeline and a set story to write to, which means that the writer instantly has less freedom. The fiction writer finds himself or herself able to create their own universe for the reader to indulge themselves in. The fiction writer is free to use their imagination to the outermost limits, and to express their thoughts on the page without limitations.
Therefore the non-fiction writer is like a drawer who traces a given picture and embellishes that picture with their artistic skills, whereas the fiction writer is like that of a drawer who begins with a completely blank page. The drawer with the blank page does not have to draw according to a traced picture, they are free to let their pencil roam and to allow their imagination be formed on the paper.
But maybe the writer who is writing according to a valid story will find it helpful to have that story. Many writers who begin writing from scratch without an actual story that happened in reality tend to allow their writing to unintentionally become off topic. Thus the non-fiction writer may use their real story to serve as a series of guidelines, just as the edge of a soccer field serves to keep the players within the area of the field. A true story serves as a container of what may become a chaos of the writing world.
If the fiction writer does not blur the door that leads to our real world and the fiction world, they will become lost in fiction. As long as the fiction writer does not do this, writing fiction does not surpass that of writing non-fiction. Just as the non-fiction writer must still be aware of what it feels like to fly from the world of reality into the world of fiction. The writer who writes in both genres will find themselves benefiting from both, just as Capote had done well in In Cold Blood.