Concerning UnrealEngine this is not a brush technique after all. It's a carving technique which is the inverse of the brushing technique ( you subtract from solid space instead of adding to empty space ). Not everybody likes this way of editing but that's also true for brushing. Now what is the catch? UT3 has a lot of rather oddly shaped and highly concave level geometry. Done with carving? Nope. What they did is using a 3D app and importing the complex concave geometry into their level. Hence the carving ( like the brushing ) has already hit their limits so they had to resort to importing triangle meshes made in a 3D app since it would have been too time consuming to do it using carving. So as you can see also the big boys in engine design resort to brute force triangle meshes for their current AAA titles.If what you say is true then how come the over-licensed Unreal engine 3 boasts unreal ed? It works alot like a modeling program (i admit & enjoy) but its still brushes. Im not saying thats the right way to do it at all - every licensee says it is. Radiant is still heavily used, Crytek has their own special editor, Valve has hammer. Gosh darnit we are pretty much covering the market aren't we?
By the way, Radiant has nothing to do with this argument at all. It's a radiosity precalculation tool. It does not edit or create geometry, it just lights it.
Like always in information technologies it's not a single solution which cuts the deal. I use also in my engine various ways of creating level geometry since some techniques work better with certain scenarios. So the trick is to know your tools and use them properly not just calling them rubbish.