Julian,

I do not have a problem with AutoCAD solids because I do not use them.  I use AutoCAD for 2D plan presentation, for which data is sourced from a 3D engineering software package.   I export the 3D surfaces to 3D ACAD for minor tweaking & layer creation (from solitary surface exports), then I export to Hexagon/AMAPI for formating & scene layout.  I export out of Hexagon/AMAPI as OBJ for import into VUE.

The models I export from AutoCAD are composed of 3D triangles(TINs), or 3D faces, which are probably the most common denominator between various 3D applications.

Any additional models I need to create, like signs or lamp standards, I create in Hexagon/AMAPI where it is very intuitive for me as opposed to struggling w/ AutoCAD 3D functionality.  It's whatever you get used to.

I realise that my workflow is different from the other people here who rely on AutoCAD for 3D modeling.  I probably did not realize just how differently AutoCAD solids are handled as opposed to TINs/meshes & 3D faces.

Have you tried creating a 3D object in AMAPI and exporting to AutoCAD to see how that comes across?  Maybe AutoDESK Support needs to be notified of the translation problems in 3D(if they care).