Again, don't blame the rules if a GM is not using them. Fatigue is an important tool for building the feeling in the Acolytes that they are being pushed to the limit. If the session is not stressful outside of combat then the GM needs to step up his game. This ruleset provides tools to do that.

Sounds like possibly FFG are trying to turn fatigue into a system a bit like the strain mechanics in WFRP 3rd and Edge of Empire?

I can see the theoretical pros and cons, but has anyone had a chance to observe how Nimble works in actual playtesting?

overall, I find that nimble is a fair skill, it is use characteristic X instead of characteristic Y to make a Z test, in this case, using agility as a replacement of toughness sounds fair overall.

The issue is Agility is already a godly stat as is, whereas Toughness has limited applications at best. People don't need to be given more incentive to pour all their XP into Agility.

Fatigue is not that great argument for toughness. The individual attributes becomes fatigued at some level no matter what toughness you have. The only part toughness has in it is the pass out threshold, and to be honest, willpower is a better stat to develop for this than toughness due to the generic usefulness of willpower against the dark gritty world of dark heresy.

If toughness is same price or more expensive than agility, you are better off developing agility and taking nimble. You will still need some toughness, and low toughness is a low hanging fruit for fatigue, but no reason to develop it beyond what you need for unlocking talents.

Part of the problem is also that wearing heavy armor is not really useful. You will limit your AB, which limits your move, your attack rate with melee weapons and many skills relying on agility. All you get is 1 point of armor in case of carapace. Power armor is a little more, but only works for 1d5 hours at a time.

Again, don't blame the rules if a GM is not using them. Fatigue is an important tool for building the feeling in the Acolytes that they are being pushed to the limit. If the session is not stressful outside of combat then the GM needs to step up his game. This ruleset provides tools to do that.

Why would the Acolytes be under constant stress for no reason? Running an investigation game, it makes no sense for the Acolytes to suddenly gain Fatigue for no reason, or to take Toughness tests when under no duress or stress at all.

Depending on what game you're running, having the Acolytes being "pushed to the limit" generally makes no sense whatsoever. And honestly, that's how a lot of people have gotten used to running Dark Heresy. As a game of investigations and cloak-and-daggering, with combat or "pushing to the limit" being relatively rare.

Feeling a pressure to shoehorn stress and pushing of the acolytes into such situations just to make a single Characteristic remotely relevant is a terrible solution to a problem of mechanics. Mechanics should be relevant to the narrative. Not the other way around.

Nimble is okay, real tanking power comes from cybernetics until endgame so toughness already lost a lot there.

One interesting tidbit is that as written now, Nimble essentially negates the use of Felling. That some weapons may have their damage code written to account for having Felling, but since it does nothing vs. Nimble, what should be an otherwise strong weapon is greatly weakened.

Should Felling also reduce Agility for the same calculation?

For my two cents I think the suggestion that the Nimble talent REPLACES Toughness with the Agility bonus is the way to go. I also think that all Eldar NPCs should have this by default.

Nimble:

Yeah, it isn't added, it replaces it. It's still ridiculous, as according to RaW it still applies if Stunned...

Page 98, Nimble: Add the following to the end of the Effect entry “Anything that would affect the character’s Toughness bonus in relation to calculating defence values (such as the Felling weapon quality) affects the character’s Agility bonus instead.”

From update #1.