I liked Benji, but it didn't grab me. It was a Great story but if it doesn't keep you engaged is it still a great documentary?
I wanted to put Len Bias in there but it was more of a story about death and drug use than a sports story IMO.
I do look at these a little differently than most. I read a lot. I only read non-fiction, and sports non-fiction makes up 90+% of that.
That said, I think you answered your own question. A great work of non-fiction is one that tells the story by pulling in the things *around* the main story to make it compelling.
Benji was more about the inner city of Chicago than basketball, just as without Bias was more about drug abuse than about the player.
It's the tertiary things that made Once Brothers and The Two Escobars so great.
The U and Broke really seemed to lack that depth.