I have a mate who gave this a go.
She had a bf, her dh had a gf. There were threesomes & foursomes on occasion.
Then her dh got the gf pregnant.
Mate was extremely jealous (she had been unable to have dc). She tried to put a brave face on it, but the cracks started appearing. Lots of rows.
Then the gf decided she'd rather be in a monogamous relationship with the father of her dc, really. & if that wasn't an option fair enough, but then she'd go it alone.
The dh - who was head over heels with joy at the prospect of fatherhood & couldn't quite grasp why his dw was suddenly miserable & angry all the time - decided it would be nicest (certainly for him) if he left his dw & set up with gf & dc.
My mate was devastated. She turned to her bf for comfort. He decided that a full on relationship with a furious, betrayed woman, who was still really in love with her xh, was not what he'd signed up to & bailed out too.
My friend is now, a few years on, happily in a new relationship, but the whole thing was an appalling mess at the time & caused huge amounts of misery. She definitely wouldn't go there ever again!
I also know one group who make it work brilliantly, to be fair, but then again they're all older, with grown up families & seem to be having a LOT of fun - without anyone taking it too seriously. Probably more swingers than polyamorists; I gather it's all about the bed hopping, no suggestion of love or LTRs.