Nennius has always suffered by comparison with Gildas (Biography; Other Works), who, for one reason or another doesn't taint his work by mentioning Arthur (he didn't exist; he murdered his brother), and Bede (Biography; Other Works) who attempted a more specific history from more tightly constrained sources. You can find a robust defense of Nennius and his "Reader" when compared with other historians in Morris*, Dumville*, and Dumville's 'Historia Brittonum: an Insular History from the Carolingian Age' (quoted in the notes to Green*).