Ideally, if you're going to take Deathmarks to go after MCs, you'll probably need more than 6, as you'll only score 3 wounds against a tyranid MC, or 1.7 wounds against a riptide, assuming you deep strike into rapid fire range.
As the Hunters from Hyperspace special rule only applies to the player turn the unit arrives from deep strike reserve, you need to be able to kill or seriously damage the unit you target in one turn of shooting. With the loss of the ability to attach Crypteks with AP2 flamers, I just dont see Deathmarks being able to do enough damage on the first turn.
The goal of the is to remove specific threats from units. As I said, I suspect the doomsday ark to be targetted by droppod Marines with meltaguns. If they can remove the meltaguns from said unit, then they would have done their job.