Radioactivity of coolant is short lived if kept pure. Heavy water expensive. Special precautions must be taken to make primary loop leak proof and to prevent loss of heavy water during refueling operations. Primary loop must be highly pressurized to achieve even high temperatures without boiling. Pure hot heavy water is highly corrosive.
Fuel suffers radiation damage.
Heat exchanges and control rod required. Heat transfer only moderately efficient. Fuel reprocessing a difficult task. Reactors must be shut down to unload and reload core. Heavy water would flush to steam in case of rupture of primary loop.