Prof Nutt, an actual expert in these matters has concluded, after taking many factors into consideration, that MDMA is pretty safe compared to many other drugs. Safer than alcohol, heroin, tobacco and cannabis according to this graph. And the subject of the thread, cannabis, appears to be much safer than heroin and cocaine. I don't think anybody is saying that any drug is 100% safe and has no undesirable effects, but we can get an idea of the relative safety of them.
Interesting article if you want to have a proper think about drug safety and drug policy. Prof Nutt looks back on drug policy a decade after his sacking.
https://journals.sagepub.com/doi/full/10.1177/2050324519898963