There is virtually no give in the suspension cords as they are usually something like Amsteel Blue for whoopie slings, or low stretch webbing as with the Warabonnet strap system.
There is some give with the hammock fabric, but that is when first loading the hammock and bottoms out with any real weight. With high quality hammocks, weight recommendations have as much to do with stretch and comfort considerations as safety. If the fabric stretches too much, you can't get a flat lay--- your butt is going to sag down, so be honest about matching your weight to the maker's recommendations.
If the hammock is properly hung and adjusted, you should find yourself laying flat with your feet slightly higher. You should be laying on the diagonal with no "banana" shape at all--- it's more like a taco :) The fabric is in contact with your entire body, so you don't have the pressure points at hip and shoulder like sleeping on the ground. You don't need much of a pillow either.
Taller sleepers need a wider hammock as much or more than a longer one. The shorter SUL hammocks like the Nano 7 tend to be narrower as well, making them even more cramped for taller users. BIAS is one manufacturer that offers wide gathered end hammocks. Hennessy has height recommendations for their models. Most of the Warbonnet models are fine, but I would ask questions if you are really tall. IIRC, the owner of Warbonnet is a tall guy.
I'm pretty much a side sleeper in a bed, but sleep on my back most of the time in a hammock. You can certainly roll on your side. I will often lay with one leg bent just for an alternate position.
I sleep longer and sounder in a hammock and free of mud, rocks, roots and bugs.
It takes a little practice to get the feel of a proper hang. The height of the tree straps and ends of the hammock will vary with the span between the trees, so each setup is a little different. Using a ridgeline is an excellent way to judge the tension on the hammock.
I absolutely recommend setting up at home, taking time to get the feel of a good hang and spending a few full nights in the hammock.
I have occasional lumbar issues that have not been a problem with hammock use.