TL;DR: it’s great. Think “Old Gods of Appalachia” and you’re pretty much there.
I have run Southern Holler in theory. I say in theory because the players went pretty far off the trail almost immediately - the incursion seems to assume they don’t go in the location described in Ring 1. My group immediately decided that was where the adventure was (and I don’t blame them, it’s a cool idea). So I mostly just picked and chose elements from the incursion and focused on the anatomy of Trophy Dark incursions (what the temptations and terrors for each ring are supposed to signal) and it went fine. Remember that the incursion is an outline to help the GM, and it’ll go great!