Originally Posted by
Saiyendra
DH generally has a better toolkit for soloing, mainly via mobility. There are lots of fights where this comes into play - Tyrant, Assault, and (mythic) Archimonde to name a few. And on Imperator for example the haste debuff from the warmages is horrible to deal with as a DK. That said DK has better damage but that's subject to change at any time and doesn't really compensate for its weaker kit (in my opinion).
And of course if you're farming totally trivial content (pre-WoD stuff) DH is vastly superior and would clear the instances in less than half the time it would take as a DK.