I think it's pretty hard to know whether poly gets credit or blame in any given situation.

For example, I know of one relationship that opened up largely because it wasn't a good relationship and then later ended. In my best guess (not very confident), the open relationship hastened the end while also making it somewhat easier to do.

At first glance, that can look like "poly ruined their relationship!" or at least "poly might mean your relationship isn't good and so don't do it or it'll fall apart!", but in this case the transition from husband/wife to friend/friend was unambiguously a good one - and to the extent the open relationship brought it to an end more quickly, it gets credit, not blame, for helping them accelerate into the crash. (Yes, this is their stance too. They were considering having a celebratory "divorce party" before just merging it into another party they had at their house)