Basically NURBS modeling works really well when you are driving much of the design from 2D profile curves, using some of those curves to generate large extended shapes and then using other 2D curves or other generated solids to cut pieces away.

The less that you are able to do things along that pattern, the more difficult things will be and the more things tend to become more suitable for sub-d modeling 3D cage driven workflow rather than 2D profile curve driven workflow...

- Michael