Single Or Multiple Authorship

Poul Anderson's future histories are written entirely by Poul Anderson whereas Star Trek is written by many script writers. These are two ways to generate fiction: basically, quality as against quantity. Anderson's output is enormous for a single author but, of course, he did not confine his attention to just one series and, in any case, could not have matched the sheer volume produced by multiple authorship.

The few Superman stories written by Alan Moore are part of the vast Superman canon but are also part of Moore's complete works and have been collected as such. Needless to say, they are way ahead of most other superhero fiction.

There is an intermediate position. Jerry Pournelle's CoDominium future history is additionally written or co-written by:

Larry Niven;
SM Stirling;
Jennifer Pournelle;
the contributors to the War World series, including Anderson.

This probably makes for unevenness in the quality of writing. There is unevenness in Anderson's Technic History because it was written over such a long period but multiple authorship almost certainly increases unevenness. However, I have yet to read the War World volumes. SM Stirling managed to pull together a consistently high quality for the Drakas! anthology.