@YIPPE re. my emphasis in bold above: the "400kg"---is clearly described as a
static load...for when the vehicle is stationary e.g. ...
not the load for travelling which is restricted to 150kg including the weight of the roof rack. The tie down rails are subjected essentially to "upwards" forces from straps/ropes securing a load... they're not designed to support a heavy vibrating downward force exacerbated by corrugations. If you unscrew the various fasteners and examine them in detail, including the substrate into which they screw, ...they are "adequate" for a reasonable tie-down task, but "flimsy" for a firm foundation required for a solid roof rack plus load.