I am using the Speckle OpenBuildings Connector from the Alpha release of the Bentley package. In my testing it seems that only the 2D geometry in any model I use is being sent, no 3D geometry, regardless of whether it is a simple 3D object or a structural component with all the associated metadata.
It seems like solids are not supported yet, but if you could transform them into a Mesh it should work. Please see here the list of currently supported geometry elements (shared with MicroStation).