The molten salt reactors have trouble of their own. One of the two experimental reactors (MSRE) form the 1960's is still sitting there and waiting for a solution disposal. It was shut down with the fuel in side. In 1994 they found that the salt was decomposing under the influence of radiation an radioactive fuel in gaseous form was moving through the reactor system. If left unnoticed longer this might even have cause criticality outside the actual reactor core. It was rather close to a nasty release of highly radioactive uranium 233. So it took a rather expensive cleanup to at least remove to uranium fuel in 2008. The rest of the reactor and fuel salt is still waiting for disposal, as there is no destination for this type of waste.
A gas is much less dense than a liquid. How would you get enough uranium in gaseous form (UF6?) together in one place to achieve criticality, when the system is designed to avoid criticality in fluid form?
Besides of that incident, the chemical separation of fuel and fission products needed to us thorium fuel turned out to be very difficult. Chances are it will be way to expensive.
You don't need to separate uranium fuel from the fission products in order to use thorium. It's a separate thing, and a benefit from using a fluid fuel; if you can separate out the fission products you can get more complete fuel burnup and much less and shorter-lived waste. Whether the uranium initially came from breeding thorium or not makes no difference. (I believe there is at least one company working on a design that just leaves this part out entirely, but those might be non-breeding designs, I'm not sure.)
Either way it's just chemical engineering, and it doesn't even seem that hard. The existence of plutonium-based weapons is one proof that chemical processes like this can be done even when things are radioactive.
Also corrosion and radiation damages are still a problem. After about 2 years of operation at rather moderate power levels the materials from the MSRE were essentially end of life due to corrosion and radiation damage.
The "E" in the name stands for "Experiment", it was never designed to be an operating power reactor or to last forever. The advocates for new thorium/molten salt designs have been talking about the need to find the right materials for new designs since the beginning. It's another problem to be solved.
Finally there is a problem surprisingly similar to the fast breeder reactors: if the reactor is made to produce enough fuel by breeding (which is the only way to use thorium as a fuel in a thermal reactor), the reactor design tends to get unstable, with a positive void coefficient, just like the Chernobyl reactor. While the small test reactor was reasonable safe because of favorable stability, large breeding reactors tend to be not.
I've heard some criticisms about it not being such a slam dunk as it's often portrayed as, when it comes to the reactivity coefficient changing with temperature. Is that what you're talking about?
If it only affects large reactors I guess it's a good thing that everyone is working on small ones.
The initial video showed a reactor the separate seed and banket fuel - thus a concept well suited for plutonium production.
By putting a blanket of U238 around it? That should work, but given that the two-fluid designs that are worked on will actively remove U from the blanket in order to use it as fuel, I don't see how any of these proposed commercial designs could be used.