Coolant doesn't react with fuel or with other core materials. Coolant will not flash into vapor if primary system is ruptured. Coolant has very low capture cross section. System can be designated with negative temperature coefficient. Any fuel can be used, including natural uranium.
Coolant is cheap.
Ordinary leakage can be tolerated. Gas turbine may be employed. Reactor vessel and heat exchangers large and expensive. Heat transfer efficiency is low. Coolant must be pressurized.