I know of a couple successful polyamorous relationships that have lasted decades, so far. I do not know that they are any less likely to last than any other type of relationship, more risk of breakups only because there are more relationships.
It does save screen time where in the books they have the whole re-birth/new bodies kind of saved by the dark one thing.
It also makes the point fairly clearly in one scene that the Forsaken are badass evil that are hard to kill.