CapnZapp
Legend
Well, not really, since for most campaigns "Underdark" is a very niche terrain that gets used only in one or two scenarios during the mid levels of your character.The flaw in my opinion by the developers was allowing the ranger to have a broad category like the underdark, where instead you would just apply a subset of above ground terrains to underground, like coast (water), desert (barren caves), mountain (spelunking), or forest (underground vegetation).
For Out of the Abyss it's a whole other ballgame. There your suggestion makes a lot of sense.
But that doesn't make the dev decision a flaw in general. If there is to be a flaw, it is how OotA neglects to make your suggestion a rule for that specific campaign!
