I once read that as you go from a single bike to a tandem bike you increase the stress on the bike by ten fold. Even though the bike is just carrying twice the people. I am not an engineer to verify that but I would not argue with the logic.
With the much longer frame and some people's affinity to pushing the limits I fully understand Bike Friday not making the Haul-a-Day folding. The folding bits on a Bike Friday are stout but add another 300 pounds to the frame---some people have a lot of stuff---and then think about the stresses of going off curbs or through potholes...