Not all "hidden faces" are hidden in the export, this bug also sadly waste a lot of pixels in the baked texture, hidden eye, teeth, tongue and other "stuff" are still there, see attached image.

The only thing visible in this scene export was the arms...

To clarify, after i did some testing, hidden faces hidden by the mesh tool etc. are hidden in the export but not hidden "objects" hidden in "Scene panel", all hidden "stuff" should be respected by the [X] Delete Hidden Faces option in the FBX Exporter...