Yes and no. It all depends on how long you leave the target.
Let’s use examples:
Scenario 1, Boss has a run out mechanic.
DK runs away, using ranged abilities while running out. Then runs back in, still using ranged abilities. Once back in the back in the rotation continues as normal. Yep, Unholy is performing well.
But then again, so is the Warrior who just leaped out and charged back in losing very little melee time.
Working as intended.
Scenario 2: Target switch to other side of arena.
Unholy switches target, uses ranged abilities while running to the new target and proceeds with normal rotation once there. Yes, Unholy is performing well.
But so is the Warrior who just charged over, not needing the run time.
Working as intended.
Scenario 3: DK player decides they are ranged.
DK player decides to stand out of melee. Maybe they think it takes less work avoid a mechanic, or they just feel like it. No, they wont be performing well.
They are not playing as intended. Damage will be trash.
They are a melee spec because they are designed to operate in melee. They lose damage when they have to leave melee. And if they are out for very long the damage becomes absolute trash.