Damage rolls in the RPG our spell is based on are not all-or-nothing like the WFB tabletop skirmish game.
Characters have a dozen or more wounds, damage is basically 1d10+strength-toughness-armour.
So three (or seven) Shadow Knives is potentially quite the murderblender in the RPG, which is the basis for our spells effect.
Really really high toughness opponents might come away unscathed, sometimes, but chances are they'll take some wounds from high rolls from a fan of knives.
Edit: also the range in the RPG is 48 yards.