You will see plenty of pics of people with UL packs, especially frameless, with the pack sagging down thier back, all the weight clearly on the shoulders.
This, should tell you everything you need to know, especially regarding claims to how great X pack is. Yeah, put it on, load it up and it looks and feels great. Walk a couple miles with it and it works its way down until all the weight is on the shoulders.
The hipbelts just arent substantial enough to support heavy loads, and most carry heavier loads than the pack works best at , part of the time.
On UL packs with hipbelts, the shoulder strap attachment points should be at the shoulder, to 1" below with the packs loaded. This lets the angle of the shoulder straps keep the pack close to the back. If the strap wraps the shoulders and goes several inches down, it doesnt have the ability to keep the pack close to back anymore.
These packs are compromises between carry capacity, comfort, and weight. You have to be willing to accept the tradeoff. Even the top light framed pack is still best under 20 lbs, OK to 25, and becoming unsuitable at 30. Lesser ones become unsuitable at only 25lbs. Regardless of what the mfg may claim. It all depends on the level of discomfort someone is willing to accept. Some people have higher tolerance for it as well.
When the pack does sag down, the load lifters can help pull it in to the back.
When it is not sagging, they can work to lift wt off the shoulders and allow it all on the hips.