Minecraft wiki entry for Badlands, associated text: Regular oak mineshafts do not generate in the badlands biomes, but they can generate into one if a neighboring biome generates the mineshaft.
Although dark oak mineshafts generate in badlands, dark oaks do not grow in this biome.
Eroded badlands continue to generate terrain in the "nothingness" part of the Far Lands. Other badlands biomes are an ocean down to bedrock layer, like most biomes.