Custom entities should be compatible with the v9.0 on-prem version of the Unified Interface.
Have you added them to your app’s site map?
If you are talking about custom activity entities, then they indeed won’t be visible from a record’s timeline.
The latest on-prem v9.0 version still has a lot of gaps for the Unified Interface and I would understand that it’s a challenge to implement it for advanced production scenarios.
Henry