Firstly, The thing is about the placebo effect, is that it is an actual effect.

Placebos are tough to account for in studies specifically because they actually DO help, for something to have medicinal properties it needs to have a greater effect than a placebo. Placebos even have been known to work when people know they are placebos.

The second one is that humans are terrible at self diagnosis / analysis, we are way too emotionally influenced, too biased, and there are too many variables to make conclusions on treating yourself. That's where scientific studies do a good job of removing biases and variability from the equation.

I'm not saying banana peels definitely don't help, because I honestly don't have a bloody clue, but I would caution telling other people about it with such certainty until there is some well researched initial evidence behind it.