Yes, but Terrain is not Mesh, you need to make it into a mesh first. Simplygon is not needed if you work with unity terrain, only for asset LOD's. If you to mesh terrains, different story but then you have no export problems with USD. And many other Objects are maybe not in a format USD understands it, so you need to either work in meshes if you use USD as source, or split things up. Other assets with Polygons are no problem, i tried the export and they work as excepted. I even exported nested Quads and planes from unity base objects i used as 3d tooltips and everything was perfectly in the USD. Tested it with Modo.