Super secure and stable. When I do the Scotland run my Roubaix sits in one of them without any problems, now admittedly I'm towing so am below Thules maximum advised speed. Here to Scotland is just under 600 miles, and normally takes 11 hours with 1 refuel stop.
I didn't see a advised speed for mine, what is it? Mines happy all the way up to 70 mph plus margin for overtaking if needed.
I was surprised Rose recommended it to me for a carbon bike. Only thing I do is give the bike a quick wet wipe below the water bottle cage to clear any grit off before I put it on.