Underground combat is mechanically exactly the same as above ground. Vast open spaces with lots of verticality. Even underground there isn't a single closed space or hallway style combat. No small caves. Enemies jumping everywhere. Instead of exploding barrels you have exploding mushrooms. Everything is well lit and "obscured by shadows" at the same time.
And any sense of danger you might otherwise have is negated by the camp button that conveniently teleports you to safety, even if you have no idea where you are after jumping down a hole.
