I'm not really sure how being physically tough saves you from what is essentially a law of reality saying "no, actually, you're dying from this leaf and you will like it" (or whatever way the flow of calamity manifests), but whatever..
As Tablos stated, it doesn't actually ignore durability, what happens is that "Calamity" is basically your Bad Karma hitting you all at once.
It's implied why the heroes aren't just instantly killed by Wonder of U and why it's sometimes easily and effortlessly fatal to others.