I can't comment on frame flex...not big enough, nor strong enough to flex a frame. I do have close to 5,000 miles on a 1st generation Spacehorse frame though. I've used it primarily for general road/gravel road riding, with the occasional tour. My tours are relatively short, and typically do not have more than 25 pounds of bags and gear on the bike. I've tried both front loading (I though the steering got a little unwieldy), and rear rack loading. I tend to prefer keeping the weight a little lower in front, but that's largely personal preference.

It's been a solid performer, and I have no complaints.