As an example of the problem, anything made of metal will become brittle. For why this might be a problem, take a look at what's in the middle of the ITER toro

As an example of the problem, anything made of metal will become brittle. For why this might be a problem, take a look at what's in the middle of the ITER toroid: you've got a stack of magnetic coils, which have to be physically braced against a colossal magnetic repulsion by a great big pre-stressed metal core. I'm sure someone at ITER has done the maths and figured out how long that core can last under operation before it becomes too brittle to keep the coils together, but I wouldn't be at all surprised if that was one of the reasons why ITER isn't designed for long-term power generation.