I´ve got a problem with slabs created from a curve: If the curve is round, the resulting slab does not match exactly the curve. The slab seems to be kind of “faceted”, not rounded. In this example, both simple geometries and the slab were created using one identical curve. Nevertheless, you can distinguish the slab in the rendering. Why is that?
This is a common issue when rendering NURBS surfaces, as rendering is always done using meshes created from the original surface. Those meshes will be created approximating the surface using faces. The more faces the more similar the mesh will be.
You can increase the quality of the mesher in the Rhino options dialog: