all the machinery used to obtain and maintain an economically viable fusion reaction. Having worked with particle accelerators and synchrotron rings, I'll tell you that stuff breaks down all the time.

The reactor breaking and taking a very long time to repair because the repairs would have to be done remotely, with robots. The structure becomes too activated for people to go inside, even after the reactor is shut off.

The reactor breaks because it's a large device operated at high stresses (power/area, neutron loading). There are many components and joints that can fail.

BTW, this means fusion will be expensive, because getting all those components to be reliable right off the bat becomes expensive. No tiny cracks in the welds means expensive quality control.