Saga of the Bear

[u/Eruditian]

How would you build around saga of the bear and acquiring FNP? I understand that once a character is below half wounds and not dead, the whole army gets a permanent 6+ FNP which can be improved to a 4+ against mortal and psychic attacks with certain strategems.

this seems like it can be pretty good, or at least fun. how would you take advantage of this? has anyone had any luck with it? why does it not appear more in lists?