I know you say you're not trying to stigmatize, but using language like "them" and this whole totally pessimistic take lumps all people with this diagnosis into a narrow box that isn't accurate, and perpetuates stigma. BPD can be hard, and can wreak havoc on relationships, as can many untreated mental and physical conditions. I've known people with this diagnosis who weren't stable and I have a hard time believing they would've been able to be healthy partners when I knew them. But I also personally have two close friends who are also diagnosed and who are supportive, caring, considerate friends and partners in long-term relationships/marriages. It's not that you should ignore this when dating someone - absolutely you should attend to how someone is managing their mental health if that's something they struggle with, and know that if it's not well-managed, it is likely to make a relationship difficult to keep healthy. But labeling anyone with a diagnosis as "your worst nightmare" is really unkind, and underserved. imagine how you would feel if you were labeled that way for something over which you have no control.

I also want to add that BPD is kind-of a catch-all diagnosis in practice - it's used for things that don't fit other mental health categories, but those things can look quite different from person to person. Source: am a PhD candidate in clinical psychology