Jealousy can be a problem, which is why people advise caution before getting into Polyamory of any sort. I guess it just depends on your outlook. I know there are at least a few married people here who are in open relationships and have been happy. Some people can handle it, and others can't. I don't think I could share a partner, mostly because of self esteem issues. My boyfriend is very big on Monogamy, so as much as I might want to, I have to respect his wishes if I'm going to stay with him.
I've watched a few episodes of the TV show Sister Wives, and they seemed happy enough until wife #4 got pregnant. Then the other wives got jealous of all the attention she was getting, even though a pregnant mate does need a little extra attention, and a new baby is an exciting thing. I don't know if they ever got over it because I haven't seen an episode since, but those are the kinds of things that need to be considered.