Which is a good argument for loose editorial control* and continuity only affecting one book, not for integration into the DCU, though.
*Although the fact it took ten years to push 27 issues isn't. Warren Ellis: a lazy jerk. "NO QUESTIONS ABOUT PLANETARY" "Finish your story, old man! No one cares about the Ultimate Fantastic Four!"
was as close to a labor of love as you can get from a big publisher; there was very little money in it upfront for Ellis and Cassaday, so they would frequently chase paychecks, Ellis by writing other books, Cassaday with his artwork for Marvel and Dynamite and, later, his film work. It wasn't laziness but paycheck economics that caused Planetary
to be spread across ten years.