I didn't claim that my evidence was more reliable, just that it wasn't less reliable. Anyway, I'm not the one making a claim about how cold showers affect a large number of people, I'm only making a claim about how they affect me.
Also, I don't contradict your experience.
I assume that some things can have positive impact on many people, and at the same time a larger negative impact on a few people. In such case, many people would recommend it based on their personal experience, and yet the experts would not recommend it. Which, I assume, could be this case. And also case of many other recommendations promoted online, or even in this thread.
After reading the other comments, I am now pretty uncertain about the rheumatism. But even so... the benefits of this therapy seem rather small, so I don't see a reason to take even small risks.
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.