During the discussion about the triplex pattern las I got to think about Felling. It's been bothering me from the start and makes it seem somewhat underpowered to me that it only applies to Unnatural Toughness.

I mean, it doesn't make much sense to me from a fluff PoV (Tough human: immune to felling; tough ork, most of his protection can get negated?) and like I said, it makes it rather weak mechanically. This is particularly glaring as TB is often much more significant than armour...

Do you think it would completely break stuff to let it apply to regular Toughness as well?
Am I missing something?

Well the reason humans don't suffer from felling is that they're already weak, which is hardly a massive advantage. Orks get 40+ toughness and at least +2 unnatural, which makes them **** hard to wound. Most human players are going to have trouble resisting more than 3 damage from anything with decent penetration.

It's kind of like pointing out the imbalance of unarmored orks and animals being 'immune' to the high penetration of certain weapons. I think that it also doesn't make much sense that penetration doesn't go through toughness. There just has to be some way to make weapons more effective against armor or more effective against big, tough, monsters without making everything one-shot a player character without prejudice.

Armout and toughness are different things. The former is a layer of something or other preventing hurty things from coming through. Toughness is how well the thing resists being injured. A blue whale has a very high TB but low armour. You can penetrate its skin with a knife with relative ease but you're not going to hit anything important, and even if you did its organs are so large that it has lots of redundancy.