Are there any advantages to making map objects out of brushes vs. making them in an 3D program and importing them?
Brushes are generally meant for prototyping layouts and for constructing very simple environments. They don’t perform we when scaling up to high-polygon environments.
Static meshes are the recommended primitive for building larger and more detailed environments. The levels in Infinity Blade and Gears of War are nearly 100% static meshes. Of course, the downside is that building a level with static meshes requires a large library of meshes or a significant effort modeling and UV mapping them in an external tool like 3D Studio Max, Maya, or Blender.