Unreal Engine has CSG also. As good practice , CSG is great to make Level base , and once you made it ,we can add details, we can add meshes . It is much easier to construct levels ,to get proper sizes and make placeholders , test game play and then change it with models.Some parts of CSG level we can leave as it is, especially floors, walls etc. , combine them with other models to get great results .
CSG Lavel base:
www.gigantgames.com/images/CSG.jpg
Add mesh to get detailed level:
www.gigantgames.com/images/Mesh.jpg