Can depend on a few things I suppose – its not necessarily a worry that the car will just collapse under the weight – these days you’d expect NCAP rated cars to be pretty tough. However it can be a measure of the strength of the fixing points – if you’re roof rack attaches the car’s roof rails those rails themselves might have a pretty flimsy connection to the car.
The other issue though can be to do with the dynamics and safe handling of the car. Landrover put pretty modest load limits on roof racks for some of their models – but thats because they’re already pretty tall and unstable vehicles- so they’re not going to recommend putting a lot of weight penduluming about so high up.
Its all pretty nominal though. The weight is only a small factor in the loads and forces the rack will experience – the wind loads and twisting forces on whatever you strap to the top of the car will often be more significant. The forces associated with the ladders I have on the roof of my car will be much more than the 10kg they weigh would suggest