Its neurotoxic like meth and coke. It might not be as dangerous and the damage is less persistent. But it's way more harmful than cannabis or psychedelics. If you want to group drugs by harmfulness, it is definitely together with the other stims.
The better criterion is the typical harm to the user. It's lower in MDMA because it's very difficult to use too much, seems to have a low potential for addiction, and is very rarely lethal. Generally usage patterns for MDMA users are much less harmful than illegal cocaine or meth users.
The studies I'm aware of were done on mice, which have a significantly different metabolism than humans. Not to mention there has been no mention of neurotoxicity from the current MAPS studies.