One small downside, as mentioned elsewhere, is that you need to add a fissile isotope to get the thorium ‘going’, and because thorium has a high sintering temperature you have to heat it up a lot to make it into fuel rods or pellets, so fabricating the fuel is more complicated.

Also, once the thorium is combined with the starter isotope it gives off a lot of gamma radiation so you have to handle it remotely - this is kind of a benefit too though, as it makes it really easy to detect thorium fuel handling so it’s hard to secretly proliferate.

However, these downsides supposedly won’t affect ‘molten salt’ reactors, which are nuclear reactors using salts of the radioactive isotopes in liquid form. Liquid fuel is easier to make than solid fuel rods, and to prevent meltdowns you can build the reactor vessel to have a plug at the bottom that melts if the temperature gets too high, so the fuel will pour into an underground vault, physically spreading it out and stopping the reaction. As a failsafe it’s so dumb and simple that it’s basically impossible to fuck up. And lastly, the reactor would use liquid metal as a coolant instead of high pressure water, so there’s no chance of a steam explosion (which is what happened at Chernobyl) making it much, much safer.

But this reactor design is still just in the prototype stage… or it was, until China just set about building the world’s first!