Turbine D has it right, but to summarize:
Blade failures--fan, compressor, and turbine--are expected now and then (bird strikes, ice, secondary damage, etc.) and so containment of high-energy shrapnel is a major concern. If the broken fragments are initially contained, they may get blown out the back end in a lower-energy state, and that is expected and manageable.
Disc failures are uncontainable, and so great engineering effort is expended to make them extremely rare. For example, while the CF6 turbine disc failures got a lot of attention, remember that this is a very widely-used (10.000+ produced) machine, some of them 40 years old, and millions of operating hours every year. So failures are indeed extremely rare, and it takes a good amount of effort to keep it that way.