Boil over is a small aspect of cooling. High heat in the engine's components can lead to significant engine damage and seizure of metal parts. When a coolant claims not to boil-over, they signify that the water vapors do not escape the system. While it's true, you won't lose coolant; your engine components will get to temperatures that they are not designed for and could ultimately lead to engine failure. The escaping steam in a boil-over scenario can be viewed as a warning sign that your vehicle will begin seeing catastrophic damage if you do not take corrective actions. The best balance is having enough cooling technology, so your engine never gets too hot.