Hmm, I've seen a lot of bikepacking/touring/ultracycling documentaries and shows (ok, a few) and there's always someone wearing sandals, usually tevas and I'm just blown away by it. That said, all of these people are putting tons of miles in and seem to be doing just fine.

I'm new to all of this and was actually thinking of trying spd pedals but am not a fan of having to clip in while going down/up technical singletrack. I personally prefer platform pedals on the mountain bike anyway. Wearing sandals while riding would mean I wouldn't have to worry about swapping shoes at camp. Plus, they're certainly cheap enough. I might give them a go and see what happens.