I drove 500 miles at 70 mph with 6 wide SUPs (32 to 34") on my Honda Civic Roof Racks with no issues at all.
I used two tie down straps on front (EACH side had separate straps looped around the center of the rack)
one looped rope around each side on the back rack.
I load nose forward and down so the wind coming from speeding down the highway, pushes the boards down INTO the racks, rather than trying to lift the boards OFF the racks.
If I was leaving the car for any amount of time, I would use locking steel reinforced straps on each side independently