In the original Naruto story, both Naruto and Sasuke are multi - dimensional characters with their own growth arcs. However, in 'naruto harem sasuke bashing fanfiction', Sasuke is often bashed, meaning his character is distorted to be more negative. And Naruto is put into a harem situation, which is completely different from the main story's focus on ninja battles, village politics, and the bond between Naruto and Sasuke. The fanfiction strays far from the original's tone and themes.
Well, the original Naruto story has a balanced view of both Naruto and Sasuke. Their relationship is central, and there are many other important elements like the different ninja clans and their relationships, and the overarching plot of the ninja world. In 'naruto harem sasuke bashing fanfiction', the harem aspect is a big departure as it's not part of the original concept. And the bashing of Sasuke means his character is not treated with the same respect as in the original. It changes the whole dynamic and feel of the story compared to the original Naruto.