It is related to A3 and complex is relative.

They region concept of a raycaster just is not as simple to handle. If you remove lighting completely, then meshes can be merged, otherwise it gets really tricky and you'd have to fix that via some complex mechanism to split material definitions into many other materials in order to get the lighting effect.

And you lose the feature to modify single regions as it was possible in A3.

But... different topic.