This list is somewhat long and varies between Mac and iOS. The app is in beta testing now, so I hope to release it soon. On Mac, two large features are support for ratings and flags and filtering, as well as Photos library support. On iOS, you will also get ratings, flags, and filtering, as well as support for working with the Files.app (instead of just the Photo library).
If you are on the mailing list, you will get an email with a detailed list of features when the app is released.