You can pirate a copy of the DSM-V and see the diagnostic criteria for yourself. It's particularly interesting to me (as I believe the field has "crystalized" more than it has right to) that two people can be diagnosed with "ADHD" but share few overlapping symptoms. Separately, fMRI evidence is still not solid on its own, it has to be paired with stronger science to be at all useful, but even so studies based on it tend to suffer from numerous problems. The most infamous case being a "study" that found brain activity using fMRI of a dead fish. That was in 2009. In 2020, Botvinik-Nezer et al. published a paper about 70 different research teams analyzing the same fMRI dataset and producing wildly different results. It's a blunt instrument but people are deceived into thinking it's really solid; it's not.