Best rim choice for backcountry long distance?

I think it's more the riders. There are still some riders (go check the "CF rim warranty" thread in the wheels section) that ride predominately by banging their rims off rocks frequently. A little air pressure would keep this from happening, or tire inserts, but for whatever reason, this is the way they ride bikes. These are not good candidates for any carbon fiber rim. There are a couple isolated locations where my primary wheelset would not be CF, due to your shoes often scooping up boulders and them bouncing off the sides of the trail and into the rims and the general rocky-ness of the location, mind you this is where riders from other locations would assure us they were "used to" gnar rocky stuff and within 2-3 runs they'd be out of action, either broken bike or broken person, very isolated situation though riding that rocky chunk in that state.

But going thousands of miles on CF rims. The rims don't care. They are stronger for weight, not for a point-impact, but for giant loading events, drops, jumps, landing sideways, etc., they are far stronger. I've raced them on about every mountain-ish race format possible. It's very rare for one to even come out of true.