I think most of the major mental illnesses are quite real. The subcategorizations get a little dicey, because humans aren't uniform enough for those more precise mental illnesses to consistently manifest. So you end up with the same maladaptive behavior internally showing up as three+ different illnesses despite being the same thing.

We lack the technology necessary to truly understand the brain in any depth, and until that day is reached its going to always be a pretty rough system we are working under relying heavily on self reporting, aka the brain explaining the brain after the information has been run through the brain.

Which also means our solutions to those illnesses is shoddy at best. Its why almost all meds are some variation of "turn them into zombie, no longer is showing symptoms underneath all the lethargy."