Drush 10 removes many deprecated APIs and functions. It's lean and clean. Drush 10 works great with improvements in the upcoming Drupal 8.8, like the new Config Transform and Exclude APIs.
Commandfiles written for Drush 9 are often already compatible with Drush 10. All you should do is expand the constraint in your module's composer.json (see extra/drush section). See this patch from Devel module. Commandfiles that aren't part of modules don't need any composer.json changes.
Drush 10 should work fine when making remote calls into a Drush 9.6+ environment. This helps during the brief period when you upgrade to Drush 10 locally but still need to make calls into a Prod instance that is still on Drush 9.