Yes, mental health diagnoses are based on symptoms and behaviors. In most cases this is very informative. I am a psychiatrist and also have a severe genetic form of arthritis as does my brother and several relatives. Like many things in rheumatology, the diagnosis is symptom based. There are some blood tests, but the symptoms are the basis. I also had cancer where the diagnosis is based on the appearance of a slide. That's all. The point in studying medicine in any of its forms is to know what informs you about a patient's suffering. Symptoms are very helpful in pneumonia but useless in ovarian cancer. In psychiatry, mood, psychotic and anxiety disorders are revealed by symptoms, while personality disorders are not so revealing. This is why you study and train. We do not use the DSM as a Bible. Only people writing articles critical of psychiatry believe that we do. The DSM gives outlines of disorders. You are supposed to know the rest. Yes, misdiagnosis is common and I have written about that with regard to bipolar disorder. This is not the fault of the diagnosis. Rather, the clinician is at fault. Clear diagnoses can be made when you understand the disorders.