And his best fiction sucks.
I'm tired that everybody is pretending that many of the great classic are any good today. It's like people eating dried days old cake, but because it comes from a great chef, they can't honestly form an opinion about it and will just praise it.
At the time, given the lack of offer and the standards they had, sure, the books were great.
But I had to read a Flaubert's as part of my French education. From today's perspective, it's boring, it's not insightful and the style is excruciatingly dated.
Same with Victor Hugo, Jules Verne, Jule Vales, Rousseau...
Sure, you have jewels that still hold very well after so many years, like Dumas or Rostand.
But otherwise, we are stuck in the past, advising young readers to dive into things because of the inertia of their reputation rather than their real qualities.
There are better things in the ancien Greek literature to read if you want more profound wisdom, better social critics in 60-80' sci-fy, and more deep and developed characters in many new york times best sellers.
Or if you really want to read French, there are Lanzmann, Pennac, Nothomb...
Not to mention we have way more than books today. Hell, there are TV shows that give you more in a few scenes than entire books of this guy.
And video games that will create more lasting impact in a few minutes of gameplay than old Gustave.
Plus if we stay in French, the field of bandes dessinées will cover a vast range of emotional and philosophical fields by the time Flaubert takes to tell you something died of pneumonia. Again.