Another user linked a Fifth Gear video testing the auto-braking systems in Volvo/Mercedes/VW cars. In it, they say the auto-braking systems only engage by braking hard at the last second specifically so you don't get blasé about it as if it were a smooth controlled braking. It's not a mode you want to use because it really feels like you're going to crash. https://www.youtube.com/watch?v=PzHM6PVTjXo