As a teenager I encountered so many people who thought they were experts with drugs because nothing had yet happened to them.
Nowadays, finishing up med school, with tons of real pharmacology and clinical knowledge, having seen lots of things in the psych wards, the ER, and stories from my wife (a physician too), I've come to realize that most people who think they know about these substances and their dangers don't really know.
Anyone can educate themselves, but lots of "drug customers", especially the most susceptible (youth + less educated), don't even have a background to know HOW to educate themselves properly concerning these things. Their knowledge is tainted by "most comments online say..." plus "and my friends who know all about these drugs..." plus "and one study I read" = so all I have to do is X, Y, Z.
Over-dosing acid by a factor of 10 will be scary for a few hours. Doing it with mdma will likely be scary for your family in the hospital.
Part of the danger of a substance is how much distance there is between the substance doing something intended and something unintended by taking a higher dose.
Part of the problem of the opiod endemic in the US is that fentanyl is so potent that due to fluctuations in the incompetent black market on it, and/or due to incompetence by its users, things can turn deadly quickly.
The other thing is how long it takes for you to notice the effects of a substance. If you get a quick result it's far less dangerous to overdose, as you'll notice when it's enough.