How much do I lose if I go hostile on them? Do I gain anything? Currently on a firestrike purifier
spoiler alert
Something about helping fucking cannibals kinda ticks me off.
You get to fight Reaper of the Lost and get his pants. That’s about it.
You gain this when hostile.
You gain faction gear, augments and the fight with the ravager, a superboss who drops a helm, if you go friendly
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You get to fight Reaper of the Lost in the Main Campaign as mentioned.
You get access to a dungeon beneath Barrowholm full of Cannibals that’s supposedly good for racking up their MIs.
There’s also a few unique lore notes that can only be found down in said dungeon as well that go into Ravager’s relationship with Mogdrogen and I think a bit on Scorv’s influence on the people living at Barrowholm.
Going hostile also means you lose the perks of going friendly with them which are the faction quartermaster (mainly important for barrowholm’s augments) and the ability to fight one of forms of Ravager.
Seems like its basically encouraging you to help cabbibals 
Personally I’m less bothered by the cannibalism and more by the murder? If they just ate people who were already dead, either for survival or to honor their god or whatever, that would be one thing. I might personally find it disturbing, but it’s not outright evil like telling you to go “rescue” groups of people and then being heavily implied to have just killed and eaten them.