You said MDMA, so I'll talk about MDMA, and assume we're speaking of users who acquire pure MDMA pills, capsules, or crystal/powder grams outright, either from trusted, reliable sources or using test kits for unsourced/unknown product. I will not consider the (potentially severe) harmfulness of popping random "E" bought off shady junkies inside a nightclub or EDM event.
First off, there's the harmfulness inherent to all chemicals foreign to one's system. MDMA, just like chocolate or aspirin, can cause allergic, paradoxical or unpleasantly intense effects on a small percentage of the population, who should not engage in MDMA usage after determining their own negative reaction to it. However, this manner of harm should not be life-threatening, even if someone's reaction is extreme sensitivity and a starter dose makes their heart pound out of their chest, a good trip sitter should be able to handle most, if not all such situations, and the worst that can happen might be a trip to the hospital, a shot of something to calm them down or to counter allergy, and perhaps some explaning to an unfriendly police officer.
Now, MDMA is also psychedelic/entheogenic, and like many such substances, it carries the potential harmfulness of triggering latent mental illness. This form of harm seems unavoidable as far as I know, short of never trying any psychedelics if you have history of mental illness in your immediate family. However, MDMA does not itself cause the latent mental illness, it merely awakens it, something that would've happened naturally at some point down the road. This harm is not life threatening, and not exclusive to MDMA, as it is shared by the likes of LSD, which is universally thought of as a safe drug, AFAIK.
Thirdly, the only "true" form of harm I am aware of involving MDMA, and by true, I mean that all users qualify to experience it--consists of the consequences, symptoms and manifestations of MDMA addiction/prolonged, multi-day binging that spills over to the next day and the next day and so on. I know a couple people who have partied on MDMA nonstop (only breaks for sleep, every couple days or so) for days, weeks and months, and the effects seem devastating--an unwavering depression, being too jaded to feel happy, extreme thinness, hellish comedowns, severe anhedonia, self-harm, you name it. MDMA addiction is the ultimate sadness. This can lead to suicide, obviously, which would be a lethal harm to MDMA.
Finally, there's the obvious harm potential of overdosing, but that is common to pretty much all substances known to man. If you pop twenty times an effective (for you) dose of most things, it'll kill ya.