The problem lies in the fundamentals of the applications. Sketchup is a face modeller, and Revit uses solids. Revit can display imported or linked face models, but, for instance, smoothing is lost (all edges become visible) and the user cannot edit the imported objects in any way. Groups and components that register as solids can be brought in a bit better via DWG, for instance, but they will remain facetted too. As curved things in Revit are achieved with real curves, a succesful converter would have to be able to retrofit mathematically defined solids over the face geometry.