There are really no cons besides the technical difficulty. It's very hard to keep ~600C molten salt contained for 40-80 years within a reactor, even more difficult to have a self-sustaining Thorium cycle (currently you need to add some Uranium to keep the cycle going).
The advantages are numerous, but safety is number one. HTGC reactors replaces the primary coolant (i.e. the fluid/gas that directly absorb the heat the reactor produces), which until now is primarily pressurized water, with pressurized gas. This means that if there's containment failure, the reactor can explode. In order to reduce the risk of it, very large and sturdy containment vessels are needed, and this is actually one of the major reasons why nuclear reactors are so expensive. MSRs operate at near atmospheric pressure. If containment fails, the liquid leaks out, cools, and solidifies instead of exploding catastrophically (e.g. Chernobyl, Fukushima). This also allows the containment vessel to be much smaller, and possibly a lot cheaper, since the issue is more with finding the right material rather than building gigantic steel and concrete containment vessels.
There's also a passive safety feature for MSRs, which is that if a reactor is out of control and excessive heat is released, the mixture of molten salt and fissile material expands and reactivity goes down, essentially a negative feedback loop. This is because in a MSR the fissile material is "dissolved" and mixed into the primary coolant, so the fissile material will be further apart and thus less likely to maintain a chain reaction when the coolant expands.
Since the fissile material is mixed into the primary coolant, i.e. it exists in fluid rather than solid or gas form, it's also very easy to separate the various fuels and fission products chemically, so the "spent fuel" can be easily reprocessed. Reprocessing a spent pebble would be much more difficult as it's an intricately designed multi-layer and multi-component solid pebble.
And of course, using Thorium as opposed to Uranium as a fuel carries its own advantages. Thorium is about 3x more plentiful, and specifically for China it not only has a large supply of it (not the largest in the world by far, but a lot more than Uranium), it's also a common and currently discarded byproduct of rare earth mining. More than that, only 0.3% of Uranium is actually fissile U235, the rest is non-fissile U238. The concentration needs to be increased via centrifugation to 5-20% for most commercial reactors (>90% for nuclear weapons and some nuclear sub reactors). A Thorium reactor, in its final form, can utilize 90% of the initial Thorium. We're pretty far from that though so it's just a theoretical advantage for the next few decades. The intermediary and byproducts are much harder to make into nuclear weapons, which reduces proliferation risk when exported as a commercial products. That's obviously a disadvantage if you actually want it to make nuclear weapons, one of the reasons this tech has been ignored for so long.
These are just some of the basics, we've got a loooong way to go to make it reality. The current Gen IV tech, particularly HTGC reactors and Fast Sodium Reactors are pretty good, long overdue upgrades over Gen I-III tech.
