I agree to a certain extent, he's a well developed character up to a certain point, the fact is Kishimoto thought with this development was enough to turn him into the final villain and the main obstacle to no just Naruto's dream to be hokage but also the main goal of the conflicts of the ninja world.He isn't really a developed character. His actions don't make sense most of the times and his goals change from left to right.
As for people hating him, I blame Kishi. If he didn't force this vomit inducing bromance the hate would've been less intense. But no, you have Sasuke do kitten here and there, and Naruto is waiting and ready to forgive him with open arms like the love sick puppy he is. People would rather wish Sasuke to die in comparison than having this whole manga centered around him, an unattractive guy the reader can't sympathize with.
Sasuke's development doesnt justify his role on the manga, and the consequence was the exageratted focus on Naruto and Sasuke relationship to make the final fight something greatly antecipated on the manga.
And to conclude it also affects Naruto, he doesnt have a consistent development on the conflict of the ninja world, he simply moves with the plot with new enemies crossing his path, he didnt had a clear objective like Sasuke, whie it focused on the bromance it damaged both main characters.
I think both were well developed to a certain extent but it's not enough consistent with the roles they both received on the war arc.