Molten salt has a lot of advantages as a working fluid over water, unfortunately the major big disadvantage outweighs all the positives.
Viz. the conditions inside these reactors would be absurdly corrosive. F salts are chemically aggressive, and that aggressive increases with temperature. That is compounded by the fact that the reactor materials will also be bombarded with significant neutron fluxes, and by the presence of all dissolved decay products in the working fluid.
We simply don't have materials that can stand up for any length of time to that kind of abuse.