I just used a pair of FCS double soft racks in Hawaii and no issue at all. Having said that it was a hire car and i don't think id want to use them on my own vehicle all that much.
I was travelling with an 11' a 8' and two short boards. At about 45mph the long boards bounced a bit but it was really just movement, not really looking at coming loose..

All in all for what i wanted i was stocked with the application..