I had only about 35kg on the rack, which the guy from Tubus admitted should not have been enough to break it. I was carrying a lot, but then I had 10 litres of water sometimes and food for a few days, plus all the usual spares you need when you tour in the third world where nothing is available.
Just to put this breakage in context, I used a Tourtec Tourlite rack in Ethiopia (where the roads are worse than anywhere else I've been) which is only rated to 20kg. I carried only 5kg less than I had on my Tubus Logo (no water on the back). It did break eventually, but after several weeks of abuse and the breakage was at a weld which was easily fixed with a cable tie (and lasted to the end of the trip). So I'm not yet convinced that Tubus racks are quite as strong as they claim.
I bought my rack in April of this year so any improvemnt has probably been made after this.