With Unreal 2 tech, Epic removed reliance on visibility calcs from the BSP tree and now rely specifically on portals and anti-portals.

BSP geometry now is used to create the basic "shell" for a map - particularly in ones that take place inside buildings or other such areas that BSP geometry is well suited to.

That said, there are entire levels in Unreal Tournament 2k4 where BSP is only used to create a huge, hollowed out volume in which to place a level that was entirely modeled in a 3rd party app.

One map in particular, though I can't remember the name, does this to excellent effect. The playable portion of the map is a sort of mountainous/rocky region set in the middle of a huge sea of lava. There are multi-textured/uv mapped and animated "lava falls", "rivers" that move through the surrounding lave sea, a couple "whirlpools" off in the distance.. The effect is gorgeous and only uses 1 large subtractive brush - actually maybe two to bisect the level, for optimization purposes. The rest is all custom models.

When you look at it in the editor, you see that it's split up into chunks that are then re-assembled in the editor.

It occurs to me that the same can be done with 3DGS to good effect. I'm planning on this kind of approach for a project I'm working on. The challenge will be breaking the map up intelligently to make for as optimized a map as possible.