If you look at mythic butcher Tank HPS, there's like 75 DKs at the beginning of the top 200 before you see a single monk. Best of the best DKs top out at over 40k HPS, the first monk on the list is still under 30k. Obviously HPS isn't the whole story since there's avoidance to consider, just saying that if you have a DK who can't keep up with you then he's probably lower ilvl, doing something wrong, or just doesn't feel like he needs to spend his runes on Death Strike instead of Blood Boil because his healers are keeping up.
As for DPS, blood DKs regularly are #1 or #2 on the meters on any fight with adds (Tectus, Ko'ragh, Imperator). They dominate the Tank DPS charts for all fights except Butcher, which is a paladin-only club. There's even talk now that dps DKs like myself should go blood for mythic Tectus because it will out-dps unholy and have vastly more survivability.
That's not to say that there's anything wrong with monks or that they can't get the job done, but the DKs Daidraco has seen are exactly what I would expect.