Thorium itself is not a fuel, its a material that is irradiated that becomes fuel - namely U233 instead of U235.
The advantage of U233 is it is not useful to make into nuclear weapons - anti proliferation. Additionally, thorium is plentiful and a thorium breeder reactor can be made using H2O moderator. U233 is very radioactive.
The downside from a nuclear safety standpoint is that, similar to Plutonium, U233 has fewer delayed neutrons, which means the control system has to respond faster to transients. The delayed neutron fraction is responsible for assuring that the reactor does not go prompt critical (a very bad thing).
This is a design problem that can be overcome with a good system, but in general it is probably less safe owing in theory to the activity of the U233 fuel and the control system considerations.
These can be overcome with a good design - for example, a reactor that uses online refueling with little excess reactivity is one that is inherently safer, even if fueled by U233.*
* Assuming one does not have a large power defect or set it up to expliode like Chernobyl