I have both Kuiu and Kifaru. For me, if I could only have one, it'd be the Kifaru. It handles weight better. It carry's light better. It's just a better pack. With that said, if all I had was my Kuiu, I'd be dang happy too. My only complaint on the Kuiu is the belt hips pads. They just don't wrap the front of the hips like a Kifaru belt does. As they only have two sizes versus three or more like most of the other guys offer. To illustrate my point, my wife is a fit women. Small tummy, (29" inches), flaring out into wider hips, (34 inches). However, she can wear both the smaller belt and the larger size Kuiu offers. Because the hip pads are so short on their belts, she can tighten the larger belt down and still carry all she wants, without it slipping.
With that said, the frame on the kuiu is major strong. It is as stiff and, likely stronger then anything any other company offers. So, the frame will handle anything you can. It is just more narrow. Which makes you think it is more flexible. It simply is not.
There are tradeoffs with any pack system. I prefer the Kifaru as stated above. But, I still carry the Icon Pro system a lot too. And, I have zero plans to get rid of it. So, my advice would be if you with the Kuiu, and are a 35 inch waist or smaller, get the large belt system on the Kuiu. So the pads will wrap the front of the hips. And love the pack system. Because, if you've never wore the kifaru belt, you'll not know any better. Because the Kuiu is more than capable. But, if you want the best, get the Kifaru and never look back.