It is lack of well defined principles how we should create custom controls compatiple with SceneBuilder designer. Some persons are trying to make ControlsFX comaptible with SceneBuilder but it is quite a challenge because of unclear instructions how it should be done.

Next think is that when ControlsFX' jar is imported to SceneBuilder it look that is also trying to load classes that should not be, e.g. Localiztion or Translation.

To read some more challenges that developer have to make custom controls compatible with SceneBuilder read ControlsFX issue tracker -> https://bitbucket.org/controlsfx/controlsfx/issue/243/all-controls-should-be-supported-by