Did you see this report on ice baths last week?
Scientists at the University of Portsmouth have found cold water immersion is no more and no less effective in helping an athlete recover after sport than light cool-down exercise.
As for why previous studies showed signs of efficacy they speculate that:
It might be that previous studies have used as a control group athletes who do nothing to warm down versus those who are immersed in cold water.
[…] We found athletes who cooled down using light exercise recovered at the same rate as those in cold water.
They also mention that ice baths pose serious health risks, but they don’t mention what these risks are.
As is unfortunately typical with science reporting, they didn’t link to the study. You can’t access it freely, but here is the abstract.
Just to add a picture to this post, here is Professor Louis Sugarman a.k.a. “the human polar bear”, who in the 1890s, brought that practice to the United States.