I think it depends entirely on whether they're willing to work on it, and how they act when they're in a bad state.
I have a partner with BPD, and she's put a lot of effort into getting better (and I've seen real progress), and at her worst she's never really acted maliciously or cruel. Still, way more extreme highs and lows than any other relationship I've been in, and I've stopped seeing them as much just because it's so exhausting.