My understanding is that more recent fission reactor designs are done in such a way that they fail in ways which cause the reaction to diminish rather than buil

My understanding is that more recent fission reactor designs are done in such a way that they fail in ways which cause the reaction to diminish rather than build up further, which puts you in a better place than, say, Chernobyl. You still have a very hot very radioactive lump of death in the middle of your busted reactor when it's all done though, and that's definitely not fun. Fusion failure modes - in current designs anyway - are far more appealing to anybody who happens to be in the area at the time.