Is it safe to delete orphaned data from my database?
This depends on what you are deleting! The Advanced Database Cleaner plugin helps you identify orphaned items, but it is up to you to decide what to delete and what to keep. Make sure to delete only data you are sure it is orphaned. If you are not sure, it is better to not delete anything, and ask for help.
And also, make a backup of your database before any cleanup. In case something went wrong, you can restore it!