There is huge evidence that shift work is bad for health. The circadian rhythm never has a chance to regulate - even someone doing constant night shifts will usually be awake during daytime on days off. As for cancer, if it is a risk factor then it is one of many. Epidemiological studies struggle to establish causality. There may be another factor associated with people who do/tolerate shiftwork that is also a cancer risk factor. As for the flight attendant, a long time spent flying could be a risk factor in itself due to solar radiation. So there are many health risks and it's something that needs to be considered when taking on a job involving shiftwork. But regular, high quality health screening and advice should be available to shift workers in particular.