The second link above suggests that De Nittis never really formed a distinctive style by the time of his death, and that assessment seems about right. He came of age at exactly the right time to become an Impressionist and spent much of his brief adulthood in Paris during the years when Eduard Manet was active and the other Impressionists were holding their exhibits. So some of De Nittis' paintings were quite traditional (if not Academic), some are strongly influenced by Impressionism and others are in synch with the Macchiaioli, a group of Italian proto-Impressionists.
Regardless of how he might be pigeon-holed, De Nittis was clearly a talented artist. Take a look: