Part of the answer here is also that most NFL coaches are going to fail. There are 5-7 new hires almost every single year. I don't think the success of Bill's coaching tree matters in any real or appreciable way, but one potential way to judge it is to look at the coaching tree of his peers. Specifically the other active NFL coaches that have had similar longevity in the league.
I did this by looking at wikipedia pages where there is a section for coaching tree. SN's point regarding how coaches can often be mapped to more than one coaching tree is a good one to consider, but for the sake of this view the names listed below are ascribed to who they were coaching under immediately before they become a head coach....
Mike McCarthy has been coaching the Packers since 2006. He has had two assistants hired as head coaches. Joe Philbin and Ben McAdoo. Fair to call both of them failures and possibly a slight against McCarthy that he has only has two assistants that the league thought enough about to elevate to head coach.
Andy Reid has been a head coach since 1999. His coaching tree is a different story. There have been some hits from Andy Reid but he also has his shares of misses. Some of these clearly could be argued that I have them in the wrong bucket, but I see it as:
Successes - John Harbaugh, Ron Rivera, Doug Pederson
On the fence - Todd Bowles
Too Soon - Sean McDermott, Matt Nagy
Failures - Brad Childress, Steve Spagnuolo, Leslie Frazier, Pat Shurmur
Marvin Lewis' wikipedia page does not have a coaching tree section, but I'll give him a success for Mike Zimmer and a failure for Hue Jackson
Sean Payton - Dennis Allen was a failure. Wikipedia credits him with Doug Marrone in his tree but Doug made a stop at Syracuse before coaching the Bills.
I'm running out of time so I'll stop.
I don't think I actually proved anything. But I do find it somewhat interesting that guys like McCarthy, Lewis and Payton who have been head coaches for over a decade has so few of their assistants hired as head coaches when compared to BB and Reid.