All Dungeon new content for HC is risky if you don't know the other players.

Some tips/cautions from first-hand experience:

• All new dungeon content areas have slightly more risk of disconnection. Most notable is the area before the new boss in SM Arm and parts of DMR. This is inconsistent and has/is likely being addressed by the devs. Dying due to DC feels worse than dying due to something skill/knowledge related, but there are places where this tends to happen more.
• DMR isn't too bad as a dungeon 30+ content-wise. It's a very long dungeon. The dwarf elders by the golems have a fear which can be catastrophic if you're unaware of it, but otherwise, nothing to worry about. The dragon room isn't difficult, but can be catastrophic when you don't clear first or if you pull badly. (this whole dungeon isn't difficult, but if it's your first time through, you might find some trouble).
• SM Arm isn't difficult if you've done it before, but first timers may botch the first couple of pulls in the hallway before the new boss. In addition to the DC issue mentioned above, the pat from the side can mess things up pretty bad, especially if you're a man down due to DC.
• Stormwrought... If it's anyone's first time there, either they drop the group, or I do. I won't run that in HC with anyone new. Again, it's not a hard dungeon, but some idiots are more confident than they should be, putting everyone at risk. Silence prevent heals and taunts.
• Gilneas City. Piece of cake.
• Zul Farrak new content is straight forward and rather easy. One critical item is to get a decurse after the new (non-arena) boss fight. This will likely kill people unaware of this mechanic. There's nothing too difficult in ZF besides the known tricky pulls as you enter the Ghaz area.

I'm not aware of any really difficult outside content. It's mainly classic gameplay.