No one has ever died directly from an MDMA overdose (as far as I know). Yes there have been indirect deaths usually from over-hydration (which causes the brain to swell, with or without MDMA), which usually happens in a rave/dance setting.
I'm sure there have been many indirect deaths from cannabis as well. Lung cancer, car crashes, falling down an elevator shaft, etc.
This is going to be true of any sufficiently powerful psychoactive substance.
If you completely ignore deaths for a moment, many report severe negative symptoms, lasting weeks or more, resulting from dehydration or overheating from MDMA. Overheating is also speculated by many researchers to be responsible for some of MDMA's neurodegenerative and neurotoxic effects.
You probably won't die due to MDMA, but if you're not careful you can certainly cause brain issues. And that makes it more physically dangerous than cannabis.
Perhaps. Personally if I drew the graph, I would put MDMA and cannabis closer together in terms of physical harm. I think they are dangerous in different ways and it's difficult to say which is more dangerous in absolute terms.
Like you say, if you are careful and informed, you can avoid lots of the dangers of MDMA with responsible use. It's difficult to smoke pot and not do some damage to your lungs, no matter how careful you are. Vaporizers not withstanding, but vaping was probably not considered in the graph.
I'm sure there have been many indirect deaths from cannabis as well. Lung cancer, car crashes, falling down an elevator shaft, etc.
This is going to be true of any sufficiently powerful psychoactive substance.