The DLC missions can be either really easy or really difficult.
For starters there are various special mirrors in the game that are part of the main plot. These mirrors have been effected by the aging war and are causing demons to appear all over Utakata. That's the gist of the DLC plot. These missions appear every other day in game until you finish them all.
As for what I meant about the difficulty in the first sentence; the demon's are strong. REALLY strong. I have Zen at near max stats and they nearly kill him in 2-3 hits. On top of that, they can't be stealth normally so you HAVE to fight them. This is what I meant about it possibly being really hard. One missed block or whiffed parry can cost you the mission.
There is a saving grace. When you download the DLC you are given an item called the Demon Blade (not sure why it's called that since it's a shuriken but whatever). This is a really useful item. As you face the demons you can see glowing red kanji hovering around them. These magical words are what keep them safe from your stealth kills. However, the Demon Blade dissipates these symbols allowing you to get that stealth kill even in open combat. The downside is that you are only given one and they are really expensive. They cost something like 50,000 gold a piece. Once you've played the game for a while you learn various ways to get multiples of items through alchemy or other cheats. Should you go this route, you can toss however many Demon Blades that you have to at one of the demons and then just run up and stealth kill them. Even works on the bosses.
So you can choose to go through these missions with difficulty or ease. Regardless of that, killing the leaders of the mirrors gets you some cool rewards. One item is a book that instantly kills anyone who picks it up (other than yourself of course). Another one ignites your sword in flames. Cool for fighting bosses, but not really useful for stealth. And another one sets enemies on fire from a distance.
I don't feel that the missions themselves are anything special. They are all the same. Kill all demons on the map. However, I'm really liking the rewards you get for defeating the bosses.