Terragen is a great piece of software, but I don't think it would be much of use for level assembly in Unity (I work as a Unity dev for about 8 years now, along with CG stuff). The main purpose of TGN is rendering landscapes and not content export, although you can export a view-dependent mesh (as far as I can tell without vertex colors, but I may be wrong here). It would be great to have an option to export tiled pieces of terrain (maybe trick microexporter to use multiple ortho cameras pointing to the heart of the planet
@Matt ? Just thinking loud).
Terragen can help you greatly with environment maps (spherical rendering - Pro version), also you can export clouds to .vdb. There is a free OpenVDB processor available for Unity somewhere on GitHub, although I can't say much about performance, never tested it.
For level assembly I suggest a dedicated asset. If you are looking for a dungeon generator, for instance, I recommend Dungeon Architect, it helped me greatly (after much customization) with my game OS Omega. Saved me about 2 months of work, easily.
Hope that helped.