Something to ponder.
First, A Kifaru has no equal in this department according to most. And, in my experience, I agree. It is truly the best pack I've ever worn. It carries light loads well, heavy loads as well as possible, and is very reliable. I'm not going to throw numbers out there like coffee house talk. I weigh loads and, I've had well over a hundred weighed pounds in my Kifaru hunter frame. My legs hurt. But, not my body, shoulders, neck, or lower back.
Second, I own a Kuiu as well. The most I've had in a Kuiu was on a 22 inch frame. It weighed 83 pounds. It was not uncomfortable giving it was 83 pounds. It wasn't as comfortable as it would have been in my Kifaru's but, and this is a big but, it did its job very well. I have since switched to a 24 inch Kuiu frame. I am certain the results would still be the same in which was more comfortable. But, I know that gap would shrink with the taller frame. Because I had the load liters engaged hard enough to really put a lot of pressure across my shoulders and chest. Something the taller frame would negate somewhat due to a greater lifter angle.
Here is the reality of the Kuiu system. There is not a stronger, more rigid frame system made then the Kuiu. To the point if it were as wide as the Kifaru's, it would truly hamper your agility when wearing it. It simply doesn't need to be that wide due to its strength. Its lumbar pad is the REAL deal. It holds as good as any I have ever worn and it doesn't take cinching your belt so tight that you can't bend over to get it to hold. The way it fits, it puts the majority of the weight right on your hips and lumbar area. Which is good because the shoulder straps are more narrow and would eat at you when you have to really cinch them down, if the pack didn't stay put. It is light, not over built in any way and, is streamlined by design. They engineered it perfectly for what it is. And, I have had zero reliability issues with either of the frames I've owned.
The Kifaru's really doesn't need critique. They are pretty much perfect. I can't find anything I would have done differently on the new version of the lite frame except, put a strap to pick the frame up with. I find myself grabbing that arrow stay that stabilizes the frame. I hope I keep on catching that habit before I yank a heavy load up and break it. I'll manage.
The difference's I think most people experience between the comfort level is the belt, the shoulder straps, and the contour of the frame between the two. Nothing compares to the Kifaru belt. Period. The thing you gotta remember too is the Kifaru was built to carry a tank and last the course. Its going to be heavier, a touch more bulky, and do its intended job of doing so better then the kuiu frame meant to offer you the weight savings, the streamlined profile, while having the ability to carry your meat out. Albeit with a little trade off of comfort. The thing you gotta decide is what is the best compromise for you.
Just know all the talk about 100 plus pound loads is not the norm for you or anyone else. Most of the time you are going to be carrying far less while hunting. And, when it becomes time to pack meat, both will get it done acceptably if you can. Good luck and God Bless