I don't think it's really iframes, but a combination of jerky animations and bad (yes, bad) melee mechanics. I rarely ever have trouble hitting stuff with firearms, because they're hitscan and don't require leading your shot. Just precision will do.
But with melee, you somehow require timing AND pinpoint precision, even when the zombie is all up in your face. If your crosshairs aren't on the zombie's hitbox at the right moment in the swing animation, you will just miss, even if the zombie is well within the arc of your weapon visually speaking. This happens a lot because the animations are very exaggerated (head movement especially) and seem to be choppy regardless of FPS.
Fast weapons like knives and fists suffer less from this, but clubs and sledges (imho) feel terrible due to these mechanics. I've gotten used to it in the sense that I can whack zombies reliably enough to make the weapons useful enough, but every fight is a reminder that so many other games did it better.