I can't seem to figure this one out.

Sometimes it seems like the spell will have no effect on an enemy, almost like it goes completely through them, or it does marginal damage to them.

Other times, it will greatly stagger them, slow them, and then a second later, their health bar drops off the face of the earth and they die.

There doesn't seem to be a huge difference in how it affects different levels of enemies. With my character Thanos Frost-Fist, who specializes in frost magic, mace and shield, and heavy armor, it's the perfect shout (coupled with Ice Form, of course, with is almost overpowered), but it seems so inconsistent at times. Honestly, so does Ice Storm.

Maybe it's because of Nords vs other enemies. It's just too hard to tell unless it's an elf, khajiit, or argonian.

I have this problem too.

Frost Breath with the reward from the Dragonborn DLC makes Frost Breath worth it. Makes it exactly like Ice Form. It's now one of my favorite shouts.