I've been playing with it for a couple days. It's ridiculously powerful. I've only messed with the Frost subclass, but it's really strong. Feels like I'm playing a paladin with double the damage and a bunch of CC / powerful healing options.
And you get everything back either per battle or per short rest too, so it is very very powerful.
There are also a few mods that add ridiculous DK weapons like Frostmourne! or if you want to dual wield, Blades of the Fallen Prince!. I haven't used those because imo they're much stronger than anything else in the entire game and it would just make you one shot everything forever.
One combo that feels unfair is that you get Death Pact, where you can sacrifice an undead ally to gain 50% of your max health. You also get to summon a ghoul, but it costs an unholy rune, so if you aren't unholy subclass that's only doable once per short rest. However, one of my companions is a spore druid, who can summon multiple undead. The dk can sacrifice those to heal as well. It's ridiculous.