I have noticed big differences between harness too. My history is as follows :
Started kiting using a Naish waist harness, I am almost 6ft 2, thin frame wide shoulders. Naish waist harness rides up a lot. I kite 1 a week in winter, 3-4 times a week in summer. Don't usually do any warm ups.
after 3 months, got low back pain from resisting the kite with my back.
4 months : lost my waist harness, bought a windsurf Da-kine seat harness
4 months 2 weeks : took a spill, put my back out a bit (took two weeks to recover). Lack of support for my back.
5 months : got a Da-kine Fusion seat harness(first model that came out) and it was great, better back support, can take spills with confidence.
6 months : leg straps ripped through nylon padding they were stitched to (no reinforcing on leg straps - crap design).
8 months 2 weeks : got leg straps restitched and hell reinforced. This harness is great, it doesn't ride up, it sits on your hips and back support half way up your back.
12 months : Harness is holding well, but I took a big spill, front loop gone wrong, kite and me hit water very hard from high up, winded, but kited for another 2 hours
3 days later : back very stiff, gone into spasm, and complete lose of lumbar spine curve in back (This particular day I did a 5min warm up routine).
My back being stiff was telling me it had been damaged, and needed rest. It took one more bending over action (not lifting any weight) to make my back go in to painful spasms lasting 3 weeks, 1 week in bed.
I am now looking at revising my harness to give me proper lumbar support(more shaped padding at the back), while I am still unable to kite as my back still isn't right.
Next time you guys get a very stiff back, uncomfortable to walk, check out the curve in you back. It'sa very bad sign.
13 months later : Walking ok, stiff when sitting down for a while, able to do normal activities, but I don't dare go out kiting for at least another month.