The problem
Vehicle body panels are thin sheet steel under paint. Conventional magnets lose much of their rated force on thin, painted steel, and they slide under shear loads, which is exactly how a roof rack or sign is loaded.
How coded magnets help
Correlated Magnetics Research's applications page says Polymagnets work much better than conventional magnets on the metal of cars and trucks, and names signs, ski and bike racks, and truck tie-down attachment systems. Its home page says Polymagnets are tuned for maximum strength on standard sheet metal, with up to 4 times the strength of conventional magnets, and its attach design note claims up to 8 times more shear strength. These are CMR's own figures.
Any vehicle-mounted load needs testing at speed, in wind and in wet conditions, and a secondary retention for anything that could become a hazard if it let go.