No moderator required. Full advantage is taken of excellent heat removal characteristics of sodium. Sodium doesn't react with uranium and thorium. Fuel can be bonded to container with liquid metal. Electromagnetic pumps can be used with fair efficiency.
Sodium reacts violently with water and actively with air.
Radiation damage a serious problem, except with molten fluid. Sodium must be kept free of oxygen. High thermal stress complicates reactor vessel and steam generator design. Fuel handling very difficult. Sodium is strongly activated by nuclear bombardment.