But I don't think it does anyone any favors to oversell the idea that it has "few" or "no" side effects -- it has mild side effects, most commonly reported in the literature are daytime fatigue, headaches and GI symptoms, and also nightmares. Mild doesn't mean it isn't a nonstarter for some people.
It's also important to remember that there are major gaps in what we know about melatonin; notably the effects of chronic supplementation are not well-studied, but earlier final awakening has been documented and this is quite commonly reported in anecdata -- I can contribute a datapoint there, as can most people in my circles who have used it.
To be clear, melatonin is great and useful, but as someone with a rare lifelong chronic sleep disorder who is intimately familiar with this substance, I think it's most useful when we're clear on what we know, what we don't know, and what actually are the limitations on a substance.
Just because downing a bottle of it probably won't cause systemic organ failure or otherwise any kind of medical emergency in most people doesn't mean there aren't tradeoffs to consider when using it, especially if you are sleep-challenged