New plugin integration method means that all existing plugin 'bridge' modules for Drupal have to be updated.
Configuration settings have been rearranged, so any custom configuration being set via a module has to be adjusted.
The highly convenient 'Styles' drop-down no longer exists, as CKEditor developers expect you to write your own plugin for custom styles that does proper sanity checking.
Workarounds? There are some, and we'll explore them.
Everything You Wanted to Know About CKEditor 5 But Were Afraid to Ask