Gluon Scene Builder
Oracle's Scene Builder
IntelliJ built-in Scene Builder
is not the same as uses Gluon's Scene Builder The former is a version embedded in the IDE, introduced with IntelliJ IDEA 14, two years ago, but without several features (like possibility of adding custom controls), menus or recent improvements , while the latter is the complete and recently updated version 8.1.1, that allows you using the latest features available.
If you check IntelliJ help, they also recommend using the stand-alone version (though they still point to the old 2.0 Oracle's version), they don't refer to the one they embed.
In case you find any issue using Gluon's Scene Builder, you can report it here.
Note: I've edited the answer to clarify that IntelliJ built-in Scene Builder does use Gluon's Scene Builder, by scanning the content folder with the
dist.jar. It is recommended having the latest version installed.