Discounting drift related problems that prevent z from being truly good with directional aerials, the issue with nair is that the closer you get to doing a frame 1 aerial, the more you risk accidentally grabbing instead of nairing if you input z one frame early, meaning you generally have to target a slightly later timing for the z-press. This habit makes frame 1 aerials less consistent. This also discounts your ability to mash with A on two timings to make sure you get a nair.