It's not that mana management is difficult, it's that DRK's skillset works against itself and punishes the player for managing resources.

Option A: Spam Syphon combos to get back your mana at a slow rate - Possibly losing threat in a raid scenario.
Option B: Use hate combo and save mana, sacrificing valuable dps in a raid environment.
Option C: Drop Grit and waste a cooldown to pull your mana back from the depths with Blood Weapon.(Even then casting grit is 1/4 of your mana.)
Option D: OT and run out of TP instead of Mana.