The first test of basic heat transfer phenomenon was made at Argonne National Lab in 1952. Pressure is lower for given steam output conditions than in pressurized water reactor. Metal surface temperatures are lower for given steam output conditions than in pressurized water reactor. Power excursion quickly damped by formation of steam. Overall thermal efficiency quite high.
Water is cheap.
Core is compact if void coefficient is low. Negative temperature coefficient. Ordinary leakage can be tolerated. Fission products are contained, not circulated. Radioactivity of coolant is short-lived if kept pure.