One thing that bothers me with studies on MDMA harm is that they tend to look only at regular/frequent users, rather than occasional users. For example, the study this post cites for MDMA causing decreased attention and general intelligence only looked at subjects who used it at a minimum twice a month for 6 months or more than 25 times over two years [1]. It's still unclear whether single or infrequent (like once every 2-3 months, which is common advice for recreational MDMA usage) use is harmful, which is important to determine for progressing MDMA-assisted therapy.
[1]: https://www.ncbi.nlm.nih.gov/pubmed/10811694