I know this for a fact, so I'll back Kaj here.

It is very challenging, but not all such pairs are doomed. I know one that's immensely stable and has been for over a decade; I knew another where the poly partner eventually couldn't take it (and got involved with me months after the breakup).

It's been my general impression. Though obviously this is the kind of a conflict that's usually kept private, so the conflict may be more common (and the perfect happiness about this issue more rare) than I think.