None of the custom units that I make in the data editor show up in the create unit tab no matter what I do.
The same goes for custom abilities, races, and everything else.
They are also nonexistant in the actual game.

Can you give me the order of your dependencies?
Campaign should be on top of the list.

Campaign or story campaign?

I don't use the story campaign file, so I don't know where it has to be...
I only encountered the bug with the "wrong" dependency list which disabled aspects like unit renaming and new unit placement...