Having been through several psychiatric assessments, both NHS and private, to work out what the bloody hell was ruining my life, I think the increase in diagnoses of certain mental illnesses and NDs is directly down to the airtime those conditions get online and everywhere else at the expense of other diagnoses.
There is a massive amount of overlap between some diagnoses and they should be diagnosed over several sessions with the same clinician, a range of information, and ideally another informant who isn't the patient.
Unfortunately, that's not how a lot of places (NHS and private) operate. They don't follow the correct diagnostic procedure, they don't use a structured interview, and with the private ones, when you book an assessment for a specific condition, they're looking at that condition in isolation of any others (nor do they consider two or more conditions that can produce the same symptoms and then some).
This sort of nuance is becoming lost and I think that's why inexperienced clinicians who are only looking for ADHD or ASD find what they're looking for instead of keeping an open mind. I had about 6 differential diagnoses ("maybes") and it took 4 years before I was definitively diagnosed with ADHD and bipolar disorder in 2015, and over the years of diagnosis and post-diagnosis I've seen a massive shift from "maybe" to a drive by clinicians to be certain immediately after a 1-hour session with a patient they've never met before, especially in the private sector.
I have that certainty now but others who don't are almost certainly being misdiagnosed and I don't think there's nothing at all wrong with them, I think they're not getting the right treatment because they're not getting the right diagnoses, and that's a real failing.
Also this article was very interesting about how they basically want to change how ASD/ADHD are handled by the NHS because there are just too many people with them now (I think a poster made reference to this upthread): https://www.bbc.co.uk/news/health-68725973