They are all similar but very different packs. I've talked to lots of people and heard their experiences. I've heard people have good luck with heavy loads with the Kuiu Pro, I've tried it heavy, I heard people have problems with 30 lbs. Same goes for Kifaru, Stone glacier, MR, Seek Outside or any of them. Fit is a bit body style dependent and personal preference.
I don't think any of them are a bad choice, but several may not be the best choice to you. Personally, I think it starts with body type and size, and the type of user you are. After that what features do you value the most (Durability ? Weight ? Pockets of a certain type ? ). All the packs mentioned I think have pretty good reliability records since the kinks have been worked out.
For instance, of the packs mentioned, I think people with more of a mountaineer build will work better with a couple of the packs, while those built more like a halfback build may have better luck with a couple of the other packs.
Users that backpack at timberline might have better success with one pack than those that spend more time in brush.
Personally, given your description of minimal and bivy, and longer distances, I would steer you towards our pack (Seek Outside) or Stone Glacier or KUIU (though I can't comment on the ultra, all I've worn is the pro). The other packs are just heavier, and less minimalistic.
Not trying to be a salesman here, just trying to state one persons opinion of perfect may not be your style. A lot depends on your body type and hunt type.
Thank you for your thoughtful response, it was extremely informative and helpful. I guess my body type is somewhere between a mountaineer and halfback, maybe think smaller stature linebacker 😉 I'm 6'1" and 210. Definitely larger framed than most of my lean and light mountaineering/climbing buddies but still love to get after a good route. I may have to order a few and try them on for size.
The current pack I've used for the past several years backpacking and last hunting season is a simple Northface Venturi 40, so no matter what I upgrade to, it's gonna be amazing 😉
