Sorry for answering my own question, but I thought of a tentative answer--- there is an uncontrollable problem, which is the unknown chemistry you will generate in the water tank. As the thing operates, you have a constant neutron and fissile material flux which will produce a mix of plutonium, uranium, fission products, pusher products (isotopes near lead), various breeder elements, and various neutron absorption products on the salt, on the water, on the plutonium, on the lead, which will eventually produce every element under the sun in some proportion.
The chemistry of all these elements in solution is completely unknown. For all we know, they will form some plutonium compound that will produce a chemical plutonium polymer muck at the bottom of the chamber. Worse yet, this sludge could flow from one part to another, producing a critical fissioning mass which could sit there, making a meltdown which could wreck the containment.
The thing will also produce hydrogen gas. It could find a way to make polymers from hydrogen and trans-uranics, and these sludges would be highly radioactive, and they could clog the pipes with impossible to clean gunk of unknown chemistry, or it could just make a standard chemical explosion with hydrogen. The unknown compounds could be chemically explosive in much worse ways, even underwater, or otherwise chemically annoying.
I don't know any way to test this other than a trial run. It might not be a problem. But if residues collect in miniscule amounts, the radioactive chemical explosions might not begin until a few years of running. The moment you have to close a plant, the disposal problem becomes a nightmare of radioactivity. Although, I suppose you could just leave it where it is.
Whether such a thing should kill the project is a matter of judgement. One could try to figure out all the chemistry (this would be an enormous RD project), or just experiment with one power plant for 10 years in the middle of Antarctica. I still think the promise is greater than the danger.