Do cold showers have real benefits or is it mostly hype?

Mixed. There is reasonable evidence for a short-term mood and alertness lift, driven by a noradrenaline response, and some evidence for reduced sickness absence. Claims about metabolism, immunity and fat loss are weak or extrapolated from small studies. One well-established finding cuts the other way: cold immersion immediately after strength training appears to blunt some of the muscle adaptation, so if you lift, save it for a different time. It is not necessary for health, and if you enjoy the alertness bump it is a harmless habit. Anyone with heart conditions should check first.