I hear of racks peeling off of cars about 3-4 times a year in the NW. Yakimas seem to come off more often than Thule, but both have the potential to go if they are the clamp and footpad style.
My two cents on this on, and the conclusion reached by those whose racks have come off, is that drilling and bolting will get you a much more solid solution.
I think the real problem is that these racks are designed to hold 165 lbs IIRC (+-10 lbs), which is just fine for snowboards and bikes. 165 lbs only gets us three creek boats, never mind the aero resistance of the things. I would guess that the total aero load of the boats and rack is probably something close to the car itself. The car has a larger frontal area, but a much lower coefficient of drag.
In conclusion, I think that a rack full of creek boats is just too much for the footpad and clamp style of mount and that you are going to need something bolted to get the performance we ask of these racks.