Fall From Grace is a demon.
Demons (and lots of other creatures) can only be injured with magic or magical weapons.
Then why is every bandit or evan rats able to hit her?
Did she stop beeing a demon (from a philosophical or metaphysical point of view, not biological) because she does not act chaotic evil?
(I *KNOW* that because of game balance it would be unfair to have a character in your party that cannot be harmed by more than half of the enemies in the game (and using a lady in a dress without weapons as tank is somewhat strange) but there must be a reason within the game lore.

She also has no problems attacing fiends or slimy stinking monsters with her bare hands, even though she should know better. When you tell her to attac a monster she should answer: "You should know that it is useless to attac such creatures without magic. I can summon lightning ant some other good stuff, you know. And I will NOT touch such stinking beast unless you give me lots of soap."
PS: Attacs from demons usually count as magical weapon what leads us to the question above.

I'm pretty sure it's for game balance reasons and nothing else. Succubi in 2nd edition can only be harmed by +2 weapons or better, which would make FFG immune to the VAST majority of weapons in the game. They also have a host of other powers that she doesn't have, like being able to plane shift, telepathy, shapeshifting into any humanoid form, or gating in Balors, some of which aren't even possible within the engine. As for the hand-to-hand thing, their Monstrous Compendium entry specifically states that they will defend themselves with their fists if attacked, but those fists aren't considered magical. However, that's a last resort for them just as it is for FFG. Besides, she's a Sensate and wouldn't be avoiding contact with monsters just because it's icky, she would be embracing it.

So she's still a succubus, but concessions of lore were made to game balance, which is perfectly acceptable.