I know that the world generation is supposed to look a little different with this option on, but it feels like world population/decoration (whatever the correct term is) is affected more than "slightly". This seems to affect any mod that adds biome decoration (again, not exactly sure what the correct term is) such as Ars Magica 2's witchwood trees and etherium pools, Immersive Cavegen's additional water and lava pools on the surface, and Recurrent Complex's structures.
These images are from instances running just NEI, GTNHLib, NEI, Code Chicken Core, Bugtorch, and UniMixins plus 1 extra content mod at most.
My experience has been with BugTorch 1.2.14.