I don't know why you would want to drag in a big OBJ brush meshI dont know if you have used *Radiant, but it is split up into multiple smaller meshes if required.
and then have to rebuild it from scratch in a proper scale (so lighting and physics work accurately with a proper meters based unit scale). Why do something wrong so you can go back and do it right later?Though you find open-source an "irrelevant implementation detail", because Radiant is open-source, it is very easy change the scale that the exporter saves the .obj file as. (Though you will need to be able to compile the thing and thus have a fair knowledge of C and C++ rather than just a scripting language).
I also used to live in the BUILD editor...Would be pretty good if it exported to .obj.
Tbh, any tool that makes it easy for a non artist to get their ideas across is good. Afterall, Maps can't be build in Unity alone.