Spanish critic and philosopher Miguel de Unamuno comments of the book, "I never took Facundo by Sarmiento as a historical work, nor do I think it can be very valued in that regard. I always thought of it as a literary work, as a historical novel". However, Facundo cannot be classified as a novel or a specific genre of literature. According to González Echevarría, the book is at once an "essay, biography, autobiography, novel, epic, memoir, confession, political pamphlet, diatribe, scientific treatise, [and] travelogue". Sarmiento's style and his exploration of the life of Facundo unify the three distinct parts of his work. Even the first section, describing Argentina's geography, follows this pattern, since Sarmiento contends that Facundo is a natural product of this environment.
The book is partly fictional, as well: Sarmiento draws on his imagination in addition to historical fact in describing Rosas. In Facundo, Sarmiento outlines his argument that Rosas's dictatorship is the main cause of Argentina's problems. The themes of barbarism and savagery that run through the book are, to Sarmiento, consequences of Rosas's dictatorial government. To make his case, Sarmiento often has recourse to strategies drawn from literature.