Low thermal heads make heat exchangers, pumps, and piping large. Hot channel factors are significant. Boiling point of diphenyl is very high. Extremely good moderator. Fission products are contained, not circulated.
Negative temperature coefficients.
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.