Adding mental benefits will, over time balloon costs out of control because the science of mental illness is entire subjective. If the patient claims they feel a certain way, well it must be covered.
You’re both right this is something to be careful about, however, the way current laws are for “parity” (mental illness gets same coverage as physical) a specific diagnosis is required by the doctor, not a description by the patient, and not all illnesses are covered, only serious ones.
For example, depression and generalized anxiety disorder are not covered, but schizophrenia, bipolar disorder and psychosis are. This makes sense as many lives could be saved, the patients as well as their potential victims in manic or delusional phases of those serious illnesses.