5twins, you are basically right, but if looking at a brake stay from an RD350, you will notice that the material thickness at the ends is doubled. Since the bolt holes reduce the cross section, the thicker material makes up for that. So an RD 350 brake stay is very much an optimised part with respect to strength vs weight. A flattened tube, like on some XS models are not optimised in the same way.
Even if it has no bearing on this particular thread: A stay for an OEM disc setup puts the stay in compression. This has the potential to buckle a flat bar stay.
A brake stay with rod ends/uniballs is useful with a floating rear brake arrangement, but has no advantage on a non-floating setup. And a floating rear brake on a rigid rear end makes no sense whatsoever..... Actually, rod ends/uniballs will increase bending loads on the bolts, as the bolt then needs to be longer. I may come across as an engineering nerd, but getting everything right when modifying brake setups is important nontheless......