[Sawrock]

Backstabbing isn't critically hitting. Critical hits represent that you've struck someone in a more-fatal-than-normal spot (whacking someone on the head with a hammer, stabbing someone in the middle of the chest with a dagger, etc). Of course, since the game has an anime theme, normally fatal blows can be less fatal than reality would dictate. I'm fine with the critical evade only working fully for heavy armor, with penalties for light armor, etc.