I've ridden at BSF, but haven't done the endurance ride there yet. I don't consider the trails to be very challenging, though there are a couple. There are some good long hill climbs/descents. Lots of water crossings. The rocks aren't a problem IMO, they are mostly sandstone and are very grippy even when wet (we rode there for a week during very heavy rainfall, no problem with the rocks), but you will likely need some kind of hoof protection (we used Renegade boots). The biggest problem for me to do an endurance ride there is the areas of deep sand. It isn't everywhere, but I rode a few trails there that have a few miles of deep loose sand, and my horses are not conditioned to that footing at all. Everywhere we ride is hard packed and rocky, so I would worry about riding fast in that deep sand and straining something if they aren't conditioned for it.
As for comparison to LBL, I haven't been there, but from all the descriptions I've heard/read it sounds like LBL is pretty flat and easy compared to the rest of KY/TN. I'd guess that you might find longer/higher elevation changes at BSF, so maximize the hill work you can do at home to prepare.
It is beautiful there, I definitely want to do the endurance ride there next year! Have fun!