As Juro said, hair is more durable than mallard; however, hair has a totally different look and action in the water. I tie and fish a lot of spey flies and low water wets and never use anything as a substitute for the bronze mallard wing. In fact, I never use hair as a wing on a spey fly because the hair acts and looks so different in the water.
I do use Krystal Flash as an underwing on some spey flies that use dyed golden pheasant rump feathers for the wing though. They are very effective if the Krystal Flash is kept sparse (3 oe 4 strands only on #3 or larger flies, 2 or 3 strands on hooks smaller than #3, and two dyed golden pheasant rump feathers stacked on top of each other, sometimes of different colors. Try a royal blue body, silver oval rib, red spey feather hackle, blue dyed mallard flank throat hackle, red Krystal Flash underwing and dyed scarlet golden pheasant rump wing. This is a very good fly when rivers are cold but clear. I tie it on Alec Jackson #3 and #1 1/2.