most non-hardcore racks will hold about 40-60kg
what will happen if you overload them depends on whether it's the racks giving up your your roof gutters

(and thus also depends on how the racks are mounted)
You can get very serious racks if you're looking at more weight - to have a roof-top tent for camping with yourself and the missus you'd be looking at something that would handle 120+ kg. Not sure if you can get that sort of thing for old subie gutters, or even if the gutters would handle that. I'm sure the die-hards would be able to weld something up that would clamp onto gutters in a more secure way, that might prevent the pressed metal from un-rolling.
I've got the cheapest pair of 5th-hand
1-inch tubing I could find (15 bucks), and they still handle 50+ kg.
RIP: stock '84 Sportswagon 4sp SR (sold after 13 years loyal service, to the wreckers for $100

)
'08 79V Forester X 5sp DR + subaxtreme bullbar, sump guard, Yoko geo ATS, kybs & 1inch kings (daily driver and go-anywhere tourer)