You can definitely do a lot of damage with DM but it's not always straight forward as other sets which is why people often think of it as subpar.
For example, Shadow Maul actually does huge damage, but it's spread over the whole animation, which takes a long time compared to some other attacks at that level, so it's not efficient to use on a single target. However it's really good when you hit multiple enemies with it, which is not always easy to do given the tight range compared to other aoes. Midnight Grasp hits like a truck, but a lot of its damage is also spread over a DoT so it affects the perception of it as well.
By contrast Shadow Punch and Smite do incredibly high damage relative to their animation times though as the lowest level abilities they are often overlooked in this regard. In the end they're pretty much comparable to other attacks of the same level in other sets. Personally I find them very useful for maximizing DPS, especially since you can fish for the crit proc from the ATO with them to follow up with Shadow Maul or Midnight Grasp for big damage.
I know many people have issue that the set's aoes are more like utilities; Dark Consumption for endurance and Soul Drain for damage/ToHit buff. Even though they do similar damage to other aoes of that level they're rarely used as such, particularly as they have long cooldowns, so the whole set feels like it's lacking in AoEs. But that said, the buff from Soul Drain is really nice since it lasts much longer then most "build up" abilities, and can be crazy good if you hit enough enemies with it. Plus with enough recharge you can keep the Soul Drain buff on all the time so long as you have enemies to fight.
Personally I'm a big fan of the set, I'm sure there are other sets that can dish out more damage on paper, but it's better then most people give it credit for, especially when matched with a defense secondary since the -tohit synergizes really well.