Goblins are already playable in vanilla, that's not a feature of this mod.
Thank you kindly for the correction!
This is only partial true (at least to my knowledge). Goblins are only playable if they have integrated into non-goblin civs (generally short history worlds have goblins unplayable).
opening up the caverns and hell (which never made sense to me they were always open just invisible, suddenly they are a problem when you make them visible),
Amostubal, this is likely because that is now a valid area for dwarves to path, whereas it was previously treated just like a layer filled with stone.