I have Tubus Cosmo stainless-steel racks on some of the family bikes and Thorn racks on my two Thorn Raven Tour bikes. And still a few racks made from solid steel rod (Karrimor from the 1970's, old French chromed steel racks that may be even older). Some of my old French racks eventually broke at the mounting points due to swaying with (excessive) shopping loads.
Tubus Cosmo were available locally, stainless steel doesn't show scratches, so I put them on my old utility bike and on one of the visitor bikes. These two bikes get a bit knocked about. I also put a Cosmo on my wife's bike, because it looks prettier than the Thorn rack and because her bike is more lightly loaded than my tourers.
Thorn rear racks are rated for 60 Kg when used with 6mm mounting bolts, 40 Kg if 5 mm mounting bolts are used, but they recommend only loading to half this weight on very bad road surfaces. Thorn are not stainless steel, so need occasional repainting, but would be easier to mend by brazing (or maybe even welding) if they ever did break.
Both Tubus and Thorn are made from tubes, so are much more rigid than the solid steel racks I used previously. Although, despite the swaying, I had no problems with the Karrimor racks that I used for many thousands of kms of cycle-camping in the late 1970's/early 1980's.
Tubus and Thorn should both be strong enough to cope with expedition use, but like Pavel I would prefer Thorn over Tubus.
