I kinda like the meat cell and other compartments on the Kuiu, but the flexible frame is what I am unsure of. This may be the reason for the warm spot with the hip belt.
I'm really just speculating/fantasizing on this topic, I'm not in the market for a another pack. I have a KU3700 on order and most likely will use it exclusively. Just can't beat the base weight savings....
Not for me. The carbon fiber "legs" for the frame are held in place by sliding into holder slots on the hip belt. The hot spots come from these legs pushing through the hip belt and sticking me in my hips. It's brutal with anything over 50 lbs. and I'm not the only one with this problem. BSeals had the same issue amongst others. Kuiu offered to try and build him a custom hip belt to combat the problem but I don't know the result of that. I'm sure they are working on a fix for future packs as we speak.
I haven't had this problem with my pack, or the slipping strap problem for that matter (I did get and install the new thicker straps though). The Kuiu suspension is extremely comfortable for me, even with heavier loads (still waiting for a super-heavy load).
I think backpacks are kind of like boots. They have to fit your body, and what works for one person may not work for another. I think Kuiu and Kifaru have gone through some effort to design packs that are adjustable, and work with a wide variety of people, but there will always be some that fit better than others.