Fission products are contained, not circulated. Radioactivity of coolant is short-lived if kept pure. Conversion ratio may be high. Heat may be taken from circulating water increasing power output. Boiling limits power density.
Radioactivity builds up in turbine.
Changes in load on turbine reflected back to reactor as pressure changes. Separately fired superheater cannot conveniently be employed. System must be designed to overcome tendency to react negatively to load increases. Fuel must be at least slightly enriched. Fuel handling necessitates complex equipments.