A quick question regarding the above: We all know that unarmed attacks cause a level of Fatigue if they score enough damage. Space Marines can be upgraded so their unarmed attacks have the Deadly Natural Weapons trait. Now the question itself is: Do all Natural Weapons count as unarmed attacks for the purposes of causing fatigue? Because if so, their applications and usefulness suddenly skyrocketed.

How so? What natural weapons are you referring to, that wouldn't also be unarmed attacks already?

As an example: Tyranids. I believe Scything Talons count as natural weapons, as does the Tail Gift fo the Gods.

Ah, that's actually a really good point. I don't think that there's a proper distinction in the ruleset, unfortunately.

Do all Natural Weapons count as unarmed attacks for the purposes of causing fatigue?

As far as I can tell, no. Sadly.