Well, it is underdiagnosed as major depression A LOT, so yeah, maybe in people with BPD it is overdiagnosed (I don't know enough about it BPD), but true bipolar misdiagnosed as depression (especially in women) is very common. I was misdiagnosed with major depression for 10 years. I have no proof, but I do feel being put on an endless stream of SSRIs messed up my brain/thinking permanently, but IDK, maybe it's just paranoia.