[Fern]

Steel Aura states it negates On Hit and On Critical effects, but Sharp Claws (the trait from Shaitan or Felidae that make you do protection ignoring Slash damage On Hit) ignore it. I'm guessing it's unintended.
 
To test, just put on Steel Aura and hit yourself with a character running Sharp Claws + any fist weapon.

[FatherCrixius]

There are quite a few effects like this; it stems from on hits pertaining to that of the weapons only. Forces weapons with on hit effects that do not come from the original weapon, such as narcus or hayabusa, to ignore the steel aura.

[Neus]

This is not a bug. The skill specifies it stops on hit effects of weapons, not other effects. Hayabusa is a set effect. Narcus, while worded in a way that it seems like the weapon's effect, is actually the effect of the status (which only triggers for Narcus). It also doesn't prevent magic bullets.

There was exception I noticed which was Roaring Falcon, which has been corrected.