BPD is a really specific personality disorder with fairly clear characteristics. While personality orders in general are somewhat controversial, and notoriously difficult to treat, I’d be interested in seeing evidence of overdiagnosis.
Depression on its own can be crippling and lethal, so complicating it with an addictive, potentially reckless or even psychotic second state is a nightmare. Mania is often misunderstood as just elevated mood and high energy, but the reality is more often of the “running up crushing debt, promiscuous and dangerous sexual activity” flavor than “spent all week coding a masterpiece.”
In extremes mania leads to a psychotic break to the point of being difficult to distinguish (even in a clinical setting) from schizophrenia. I knew a lovely guy who worked on motorcycles for Harley, and he was always troubled, classically bipolar, but refused to get help because he loved the highs. Then one day I got a call from his wife that he had spent three days “talking to Jesus and the devil” and pleading the case for his soul. He had to be committed for observation, which turned into weeks and then months of inpatient care. Bipolar disorder really is no joke, and the tendency for people with the disorder to struggle with substance abuse doesn’t help. “Dual diagnosis” is a phrase that portends a world of trouble, especially if you’re not wealthy enough to avoid various aspects of the system.
The last point is that BPD typically improves with age, while bipolar disorder typically becomes worse.