I am not a historian, nor is english my native language, so I hope this answer is up to standards, and that it doesn't insult the english language too much with its imperfections. Here it goes.

Sarmiento was a man of his time, and as such he must be judged. Argentina had come a long way since declaring it's independence from Spain in 1816, but is still hadn't, by the time of Sarmiento, been able to have its internal conflicts resolved. Basically, the country was split between the "Federales" (federals) and the "Unitarios" (unitarians). The main difference, to put it briefly, was on how political power would be excersized: While the "Federales" wanted to give each "Pronvicia" (state) its own pseudo-independence, allowing them to dictate their own constituions, elect their own authorities, and have sovereignity over all of their territory, the "Unitarios" wanted a centralized power that dominated all of the argentinian lands. For reference, the USA is "Federal" (as we all know) and smaller countries usually tend to be "Unitarios", since it is easier for one political power to raise to prominence over all of the land. Argentina is, incidentally, a very large country, so it was difficult for Buenos Aires, as powerful as it was, to extend its tendrils to the provinces that were farther away from it.

Sarmiento had embraced fervently the unitarian cause, even if he was recruited for the Federal army at the age of 16 (the federals controlled the government back then, with Rosas as their strong man, who excercized a presidential dictatorship). Sources cite that when he was at the federal army, he would often criticize it, and at one time, after presenting a complaint asking equal treatment for him, he was cited by the governor of his province, and was held in... contempt? (not sure how to say this in english... he was found guilty of not respecting the grade of the person above him) and sent to prison. In 1831 he finally went into exile to Chile, where he would work as a teacher. He would go back to Argentina in 1837 after the government allowed him to, at the request of his family, and it was frm here on that he would rise to prominence, first in the cultural circles, but soon he would go into politics. His discussions with Alberdi show us that he was very much (as was Alberdi) a Liberal, as well as a constitutionalist, a democrat, in favor of immigration, and public education and progress through intellectual development.

This clashed heavily with the "Gaucho" (something akin of a cowboy, for those who don't know) lifestyle. Gauchos tended to live atop their horses, going from one master to the other working on the fields, and getting everything they needed from them. They slept wherever they were that day, eat charred meat (the traditional argentine "asado") and had little care or use for any kind of formal education. This was very convenient for many of the ruling elites, since it was a cheap, easily controllable and highly specialized workforce, but it was hardly of any "use" for any country that wished to enter the new era of science and industry that was blossoming in Europe.

Sarmiento had a background that drove him to think that education and culture were paramount for any society (he had been a teacher, and had been in touch with many artistic circles both in his native San Juan and during his exile, in Chile), had written for many newspapers, rubbed shoulders with the intellectual aristocracy of the time (Alberdi and his pals), and dreamt of a country that embraced science and progress. Clearly, the gaucho was not fit for this.

You can see how much he despised them in this letter he wrote to Bartolomé Mitre, a prominent political figure of the time in Argentina: (this is a rough translation I am making right now, so please forgive any eventual mistake) "We would like to set those savages, for whom we feel, without remedy, an unbeatable repugnance, apart from every american social matter (...) do not try to economize on 'gaucho' blood. This is a fertilizer that we need to make use for the benefit of the country. Their blood is the only human thing that those savages have"

According to Felipe Pigna, Sarmiento visited between 1845 and 1847, on behalf of the Chilean government, several countries including most notably Uruguay, Brazil, France, Spain, Italy, Germany, Switzerland, the US, Canada and Cuba. Sarmiento had ended up in Chile, again, after being exiled, again after being thrown into jail for clashing with the Federal Government, in 1840. Pigna states that there he went full on cultural, writing for "El Mercurio" (which is still today the most important newspaper in Chile), and was named Minister of Public Education. He also founded the Faculty of Phillosophy and Humanities in chile, and after this he was given by the Chilean president Montt the task to study foreign educational systems, which would lead him on the travels previously stated.

I can speculate, based on the information stated above, that Sarmiento saw the federal party and government that was ruling Argentina as an insurmountable obstacle for the developing of the country on the way he thought it was best. He had been exiled to Chile twice, and on both opportunities, he had been able to function as an intellectual and journalist, showing his capacity, and even being empowered to enact actual reforms. In Argentina, however, he was basically thrown into jail every time he opened his political mouth.

When he wrote the Facundo, this, in my humble opinion, showed. Rosas was the tyrant that wouldn't allow intellectuals like him to flourish. He would favor the rich landowners, who in turn, fed from the cheap 'gaucho' workforce.

Sarmiento, after a long time in exile, and after a series of events which would derive too much from the topic, but that ended up placing him in power; tried to build a few communes for small landowners ("chacreros" as they are known in Argentina, being a "chacra" a small plot of land where a family works for itself) and even after a resounding success, got a letter from the Union of Landowners that stated that they "didn't see in good eyes implanting such colonies in places where the industrial agriculture was already rooted". I personally speculate that such a letter, coming from the most powerful sector of the argentinian elites, sounded as a threat.

Events like this one led Sarmiento to despise the elite, aristocratic, rich landowner families from Buenos Aires, as you can see in this letter (again, my translation): "Our landowners know squat about the matter (the matter of cultivating the land through small landowners or "chacras") and they'd rather build themsleves a palace on Alvear Avenue (a most aristocratic, even today, neighbourhood in Buenos Aires) than get into business that would fill them with problems. They want the government, they want us that don't even have a single cow, to contribute and duplicate and triplicate their fortune to the Anchorenas, the Unzué, the Luros, the Duggans, the Cano, the Leloirs, (all of them, aristocratic families, most of them still are today) and every millionaire that spends his life watching a cow give birth. This is the state of the matter, and since the Congress is also filled with aristocratic landowners, we will hear tomorrow the same old song, the sound of the guitar below the shadow of a tree in the Pampa, and shacks made out of straw". Here, Sarmiento clearly states that he didn't believe that the ruling elites that had rose to power with Rosas in office, had any interest or wish to industrialize the country (they are content watching a cow give birth) and, to him, civilization went hand in hand with the urban, the cities, the european, schools and education.

He clearly didn't see progress through the gauchos, and didn't see any possible change while the rich elites, most of them staunch federal supporters, benefitted from the Gaucho existence.

So in short, yes, it is my personal opinion, based on what I just cited and speculated, that Sarmiento had a very "Black and White" opinion on Rosas and the Gauchos equaling Barbarism, and education following the european model (which he he had been able to witness first hand while working for the Chilean government) as the only means to true civilization.

Hope it helps, I would be happy to expand on any points, or provide further sources, if necessary. There is a rather interesting letter that a gaucho turned into popular leader, called "Chacho" Peñaloza wrote to Sarmiento, and his response to it spoke volumes in regards of what Sarmiento thought of gauchos, but I don't wat to disgress too much so I left it out.

I am thrilled to see a question regarding Argentinian History on this subreddit!

Cheers!