I just checked and the straps I use are Polypropylene ones, specifically Coghlans Sleeping Bag Straps. The buckles are not as nice as Fastex but the straps are light cheap and work well.

I doubt that most folks abuse them like I do so buckle breakage is probably a non issue for most (I know that my touring and backpacking companions use the same straps without breakage). Breakage has always happened when tightening (over tightening?) and not while underway and the place they have failed is the part where the strap is permanently attached to the buckle, so that just tying it to the buckle is a quick fix, but I generally carry one spare strap anyway since it has also been handy to have another one when carrying some impromptu load now and them.