Coolant would not vaporize to any great extent in case of rupture of primary loop. Coolant doesn't become radioactive. Ordinary leakage can be tolerated. Corrosion of metal surface is negligible. Decomposition products of diphenyl may be deposited on fuel elements and other heat transfer surfaces.
Fuel suffers radiation damage.
Fuel reprocessing a difficult task. Heat transfer not so efficient as in water system. Hydrogen gas is produced. The extraordinary heat transfer properties of liquid metals make them attractive reactor coolants for both thermal and fast reactors. In addition, liquid metal systems operate at low pressure require the minimum of pumping power and are capable of operating at the high temperatures required to generate steam for modern turbine generators.