Lots of us use frameless packs. I have a GoLite Speed (before the redesign) and a homemade frameless one. On both, I can stuff my gear well enough that I don't need a pad as a framesheet. I've started carrying a torso pad but mainly just use it as a sit pad...prolly leave it behind for most summer trips in favor of a simple sit pad sized just right for a pack frame. But a frameless pack is easier to use w/ a hammock than a tent, IMO, b/c there are no poles to worry about.
Re: back issues...it depends on your issue! A good fitting pack is the most important thing. In general, frameless packs take more consideration when packing, and I'd imagine it's more of an issue of you have back problems. You may be able to find a frameless pack that fits your needs...only one way to find out! I wouldn't rule out frameless packs simply b/c I had back problems, but I'd be more careful when testing out new ones and take shorter trips until I found one I liked.
And I second the suggestion that a pack should be one of the last items you go UL with. It's easy to carry 15 lbs of UL gear in a heavy pack rated for 50 lbs. It's not so easy to carry 40 lbs of normal gear in an UL pack rated for 25. And bulk is an issue there, too.
Especially in summer, a hammock can save you quite a bit of bulk AND weight for the level of comfort...a tarp+CCF pad is much bulkier, while a tarp+inflatable pad is heavier, than a hammock system for the same summer temps. Plus you get bug protection! And you can always sleep on top of your frameless pack for a little extra insulation if needed...can't do that in a hammock with a frame pack! Not safely, anyway...
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