I remember reading about an area, I can't remember if it was somewhere in China or another country in the region, where there weren't enough women to go around. Therefore, brothers would often share a wife between them. Any children born of the woman would call the oldest brother "father" and the younger one "uncle."

It can work, but yes, there's probably a reason why it's rarer than polygyny, and I would assume that inability (until recently) to determine who was the biological father of any children born to the wife would be the major reason.

EDIT: I just looked it up, and it's Tibet, and the reason it is practiced is not a shortage of women but a shortage of land. There's not enough land to split between all the sons of the family, so they all stay in the same household, sharing a wife between them, rather than each one starting his own. Not knowing who is the father of any children born to the wife isn't as big of an issue since all the husbands are related anyway.