I wouldn't say aluminum has 'no' flex, but when it does, you risk the material cracking. However, being it's a cylinder, it's doubtful. I think the weak link is (and should be) the u-joints, although better have a loop should the shaft come off in front (yikes).
Nice looking piece. A little pricey, of course, but once you factor in the time, sometimes it's worth paying up front. Just curious why they didn't also make the front piece, but it's good to eliminate one of the flex points.