Bitterblack isn't really a traditional dungeon. The floors escalate in order, but the rooms don't. Almost every room in the dungeon has 2 configurations, another depending on your actions and some have many more because they're arenas. So you can go into a room and find a bunch of Goblins...or a squad of death wolves. It'll always be one of the two and the one is harder to deal with depending on your class. There's hub room super early where you can potentially encounter a zombie dragon early, and another where you find a chained up gore cyclops you're not meant to fuck with. If you're intent on killing everything you meet then you're going to need Bitterblack Gear, or just run passed things that you can't. Many of the enemies, unlike the vanilla enemies, are also meant to be taken on by specific damage types: they're incredibly staunch against some damage types, but take a lot more from others such as fire. On top of all of that, the upgrades you get from Bitterblack (not just gear, also skills) make you a shitload more deadly: without those upgrades you are considered undergeared. The first thing people do in Bitterblack is rush to get the first weapon upgrades there.
All in all Bitterblack is much more advanced than the base game. And once you kill the final boss there...the entire place takes off the training wheels and it gets crazier.
That being said, there's nothing in there you can't solve with the liberal application of blast arrows.