3D models - dumb or feature-based?

There can be substantial differences between 3D models. With so many 3D CAD applications, 3D libraries are often shared and converted from/to different file formats. During such conversions you will usually loose some "intelligence" of the 3D model.

So you can find many 3D blocks as wireframe only (very limited use), many 3D blocks as meshes (surface facets, no volume data - typically from visualization/rendering software) and many blocks as dumb solids (no features).

The intelligence of models created natively in parametric applications like Inventor or Revit is important when you need to modify/edit the 3D model. Depending on its intelligence, it can be as easy as changing a single numeric parameter or as difficult as redoing the model from scratch. Also the size of the model can depend a lot on the way how it was created. Defining a geometry and features using the native functionality of a parametric modeler leads to small, fast and effective models. Importing the same geometry as dumb objects makes larger, slower and clumsier models.

So make sure your 3D DWG block is really a solid object (not just facets), make sure that your IPT or RFA blocks are really created in Inventor or Revit, not just imported from AutoCAD DWG, IGES or other general (dumb) formats.