I have a packframe with a shelf. It can carry a whole lot more than I can lift. I tried to get two elk hindquarters out at once and it was too heavy for me to walk across uneven ground. (thats a joke, I could barely stand up)
I am currently using a backback but seems to me they are not designed for carrying unwieldy clumsy things, like hindquarters or midsections. When I was going into the mountains I would wear my packframe and a small fanny pack. And yes, one time that proved to be a problem....I needed more water, more food, a blanket...
My packframe is ancient-I mean mid 1980's. I just checked straps and things--all okay. It is welded tubing very very light. Adjustable in about 8 directions seems like.