Remember this is not the Unreal Warfare engine and its not based on static meshes or other large high poly objects. Its dependent on CSG.
Constructive Solid Geometry (CSG) is a technique for defining a detailed space by building it up gradually with simple shapes. This is what the Unreal Engine uses to define the basic Unreal World.
One of the big differences between making maps for Unreal Tournament and Unreal Tournament 2003 is that far less CSG is used to create its levels. In a UT2003 map, space is hollowed out (and occasionally added) of the solid world using CSG and then populated using Static Meshes.
I use to work with Subtracts and Adds as long possible and use Semisolids only for brushes that it makes sense for.