Some rucksack manufacturers are obsessed with making their sacks lighter and lighter, almost to the point where they're little more than bin-liners with webbing straps.
Don't fall into the trap of sacrificing the support of a good harness which will ultimately be more comfortable, in favour of saving a few grams.
There are some where this is the case, and for the use case they are intended for that's fine. If your a SUL thru hiker with down quilt, a poncho tarp, and next to nothing else, if you're base weight is <2.5kg, why not use a very light weight simple backpack?
My Exped Lightning 60l Pack weighs in at about 1.1kg with it's extra pocket on the front. It's basically a dry bag, with two mesh side pockets, and a really good suspension system. I don't need anything more, and half the time I struggle to fill it. I've used it for a few trips and It really is the most comfortable pack I've ever used. I kinda wish they did a 30L version.
My current day pack I'm testing is 360g for a 15l pack. I think I can actually get everything I need for an over night in it. Will be testing that this week...
J