Balderdash - I admire the fact that you have a seemingly stable friendship with someone who has HPD. That said, the key breaking point that destroys HPD relationships is the cheating and lying. Friendship is not exclusive the way a relationship usually is. Friends can take breaks from each other for days or weeks if needed, with no harm done to the relationship.
I know exactly what i can and can't trust him with, and I've learned to tell when the rules change.
And the things you can't trust him with are things that you can accept being lied to about. The rules that change don't affect your closest intimate relationship. That's why you can make it work - it takes strength to put up with the histrionics and the drama-queen persona but they're manageable if the friendship is worth it to you.
The only analogy I can think of is imagine you'd go out to dinner or to a club with your awesome friend, and occasionally he'd turn around and stab you in the face for no reason. Not maliciously, just as part of his disorder. It'd be quite difficult to maintain your friendship then, hmm? Their behaviour towards their current partner is like this.
HPD sufferers can be great friends, and are often plenty competent in the rest of their everyday lives. The reason we here are somewhat negative is that most of us are 'hpd relationship survivors' and that (as part of their disorder, obviously they 'can't help it') they've suckered us into committing to a relationship, and then psychologically and emotionally tortured us for months or years before eventually discarding us, only to show up and try to reel us back in the moment we start moving on and trying to live a normal life.
You say we assume they're all the same - how many of the stories here have you read? They tend to be disturbingly similar, and that's what leads people to stay on this forum. We've each thought our own situation was unique, tried to rationalise or excuse our partners' behaviour, and probably at some points doubted our own sanity due to their gaslighting and facile manipulation of the truth. Then we stumbled on this forum and saw our own story written dozens of times over by total strangers. There are individual differences, yes. The similarities are overwhelming.
I have no doubt that the emotions my wife feels for me are real at the time. They're just incredibly ephemeral. She can tell me "you're the most amazing man in the world, I'm so lucky to have you, you're so wonderful" and ten minutes later demonize me for answering a question with the wrong inflection in my voice, or for making some small random noise that she demand to know the meaning of. I understand that she's not evil or hateful, she's a genuinely good person, one whom I love. I still feel we have a future together and hope that with help, she can overcome the worst of these traits and that our future can be smoother.