If you're a seasoned Drupal module developer, or even a relatively new one, it's hard not to like the fact that, starting with Drupal 8.7.7, it's possible for a single version of a module to be compatible with multiple versions of Drupal core. Suddenly, maintaining your module became way easier. It's noteworthy enough that the process of making a module work with the Drupal 9 release was incomparably easier than any previous major version upgrade. But beyond that, you could actually maintain a single version for both Drupal 8 and 9, or both Drupal 9 and 10. How great is that?
But - and there always is a but, isn't there? - it's not quite so straightforward. There are some significant tradeoffs to sticking with a single release branch for two major versions of core. I'll look at a couple here - deferred refactoring and missed improvements - and ways to mitigate them.