With the introduction of Abominations, I'd say that the Swamp is now definitely more difficult than the next biome, the Mountains.
Skeletons are easy and Draugr aren't that tough, either, especially since they're half-blind and only spot you when they're very close to you. But both spawn in fairly big numbers. The Leeches aren't a problem unless you're forced into the water. Blobs are easy unless they surprise you while you're fighting something else. But what the Abomination brings to this equation is durability. It takes time to kill. Now suddenly the Swamp terrain introduces a big challenge; you probably need some space to deal with the Abomination, so it will push into those other enemies and into difficult terrain where you find yourself running through leech-infested waters.
I'm not really complaining, I like the game to get rather more difficult than easier, but I just find it interesting that the 3rd biome (Swamp) now definitely feels more difficult than the 4th biome (Mountains).