while self diagnosis isnt smth to do lightly at all, and requires significant and lengthy research, i think decrying it outright in both psychological and physical cases ignores several scenarios where people just cant access treatment - for financial or social reasons.
while i cant speak so much for physical diagnosis, i have heard accounts from chronically ill ppl abt doctors dismissing their later-proved-right concerns for years. but in terms of psychological diagnoses, there are all sorts of barriers to diagnosis for various groups; people of colour and women often get biased diagnostic treatment. people of colour are more likely to be dx'd with odd or adhd when theyre autistic, and women go unnoticed for adhd and autism in large quantities, and often get misdiagnosed with bpd instead. also, ASAN, the leading organisation in america for autistic advocacy, explicitly supports thorough and well-researched self diagnosis. (ik this is v autism focused... im autistic, sooo..)
so while medical diagnosis is always preferred and self diagnosis never ideal - and youll find this is the feeling among ppl who have self-dx'd as well - there are cases where self / informal diagnosis (diagnosis agreed by medical professionals or other experienced individuals that doesnt go on official record) is not only necessary but sometimes the only option. if done responsibly, it can be greatly beneficial to certain people.
this isnt to deny of course that sometimes its done irresponsibly. i just think it should be made clear that there is a time and a place and a right way to go about it.