I routinely ride my folding bikes off-road, but all my bikes have steel frames. The bikes have too narrow tires for some sandy paths but then it is normal for life to be imperfect. When it makes no sense to insist on riding a given stretch, I just walk to save energy for other parts of the route. The steel frames have been chosen intentionally considering banging up of the bikes in transport, but presumably the steel helps in the off-road endurance too.