Shoulder harnesses are (or should be) designed to break (or slip, it they have inertial reel) at the point where expected internal injuries inflicted by them would be greater than those suffered by head hitting the wheel. Successfully tested 20 or so years ago by a friend who borrowed my car (a domestic variant of Fiat 128) and had a head-on collision with a van. His speed was about 110 km/h, van's about 50. The friend suffered a finely balanced combination of face and internal injuries; had the mix tipped any way (that is, had the shoulder harness broken any sooner or later), he would probably not have survived. Rehabilitation did last a year. (Van driver did not wear a belt and flew through the windshield, but also lived to tell the tale.)