Così fan tutte has always been the problem child of the Da Ponte operas. Critical favor has elided it in favor of Figaro and Giovanni , it's more illustrious siblings. This is partly because Così 's tone (or message -- or moral -- etc.) has been so difficult to pin down. This is a story of two young couples who have their sense of self and beliefs in love systematically dismantled. But the ending is happy, with all being forgiven and a double marriage!
ruminations etc.