Flex can be a good thing...
Watch the frame of a big truck going over a really bumpy road... It flexes! In fact, it's intended to flex, and if didn't it'd rapidly crack and be torn apart. Now let's apply this to airheads... If you brace and reinforce the frame of an airhead, it gets much more "buzzy" as it's transmitting engine vibration rather than absorbing it. That begs the question: Was the airhead frame intentionally designed to flex? Probably, especially given that it was designed by a Brit who had previously worked for Triumph/BSA. British and Continental steel tube manufacturers have always made frame tubing in various guages, etc. so bicycle frame builders could mix and match them to get the best combination or rigidity and suppleness, and those same manufacturers supplied the motorcycle industry too.
Thus I suspect the airhead frame and a lot of other bike's were purposely designed to flex somewhat. One notes this by the change in resonant RPMs in an airhead fitted with a sidecar subframe, even without a sidecar mounted. Thus the subframes we're fitting in fact may or not be strengthening the outfit, and multiply the number of subframes times the possible locations of mounts and sidecars to be mounted and we'd need supercomputers to figure it all out.
Perhaps that why some outfits get by fine without subframes, and some break frame bits even with subframes?