Citing a portion of the link by electricpete,
The proposed salt mixture FLiBe, contains large amounts of beryllium, a poisonous element. The salt in the primary and secondary cooling loops must be isolated from workers and the environment to protect them from beryllium poisoning.
FLiBe also has lithium salts, which can be psychoactive drugs. These also must be isolated from workers and the environment.
Hot fluoride salts naturally produce hydrofluoric acid in contact with water. When cool, fluoride salts are nearly insoluble in water. Although HF generation would be taken into consideration in the reactor's design and shutdown/decommission processes, this hazard needs to be addressed in emergency situations that damage all five levels of the reactor's containment while the salt is hot.
As with any nuclear reactor, there are the expected needs to contain the radioactive core, which is dangerous to life and health.
There is also a need to manage the waste, which is still very radioactive, even though it is hazardous for a shorter period.
Notwithstanding the hazards mentioned, this technology is being used to some extent in India. It may be an old technology which deserves a fresh look.