I could have sworn I read somewhere either on this forum, or possibly a different PF forum that Immunity to conditions that stack into worse conditions, the way fatigue stacks into exhausted, for example, merely render you immune to the EFFECTS of the condition, not the actual condition. So you could hustle and be immune to the effects of Fatigued, but if you then didn't sleep for one night, you are now Exhausted. Resting sufficiently would reduce the Exhausted to Fatigued and you no longer suffer any penalties. Is this notion supported by RAW or Dev errata anywhere, or was it an asspull to keep rage cycling from getting too out of hand?
TL;DR: Does immunity mean you can't be fatigued, or you don't take a penalty for being fatigued?
I could have sworn I read somewhere either on this forum, or possibly a different PF forum that Immunity to conditions that stack into worse conditions, the way fatigue stacks into exhausted, for example, merely render you immune to the EFFECTS of the condition, not the actual condition. So you could hustle and be immune to the effects of Fatigued, but if you then didn't sleep for one night, you are now Exhausted. Resting sufficiently would reduce the Exhausted to Fatigued and you no longer suffer any penalties. Is this notion supported by RAW or Dev errata anywhere, or was it an asspull to keep rage cycling from getting too out of hand?
TL;DR: Does immunity mean you can't be fatigued, or you don't take a penalty for being fatigued?
It depends on the wording of the ability/spell/etc. Some examples:
"suffer no penalties for being fatigued" = you are fatigued, but you don't take the penalties, so your push into exhaustion idea might work.
"immune to the fatigued condition/cannot be fatigued"= you are never going to be fatigued again, so more fatigue can't make you exhausted (things that make you go straight to exhausted, however, probably will still work).
"removes the fatigued condition" = you are no longer fatigued. Another round of fatigue will make you fatigued again, but you won't become exhausted.
"suspends the fatigued condition" = you are not considered fatigued currently, so another round of fatigue won't push you into exhausted. Whether a second dose of fatigued would make you fatigued depends on whether the ability/spell says "suspends conditions the target is suffering when it's activated" or says "any condition during the duration" or some such.
Immunity to fatigue means you aren't fatigued. Anything that causes fatigue normally, doesn't cause it for you.
But you aren't necessarily immune to exhaustion. Take the Waves of Exhaustion spell.
Now, for the specific question of multiple sources stacking into exhausted from fatigue....I think immunity to fatigue would prevent that.
And as far as an asspull, I haven't seen that stated by any developer or FAQ. Rage cycling is still a thing, though the new Unchained Barbarian has been written specifically to prevent it. Though mostly rage powers that were only useable once per rage are now gone because they just didn't do enough.