Since moving development to Drupal 8 for all our new builds one of the modules that we relied on heavily for many of our Drupal 7 builds has been in a bit of a state of disarray in it’s Drupal 8 version. That module would be Rules. What was once a powerful tool for creating conditional reactions to specific events on your site is now something I actively avoid having to use on a build. Documentation is sparse, many features are missing or barely work without the help of multiple patches and the UI is just plain unintuitive. All of these issues leave you taking shots in the dark to achieve the functionality you’re looking for.