ROTJ doesn't put as much value in the main cast of characters versus other movies. They're still incredibly valuable, but a lot of the battling and fighting feels trivial compared to happened in Empire.
For example, in Empire, every non Empire character (as in, they aren't in or involved with the inner workings of The Empire) gets dealt a terrible hand, and there's a lot of tragedy mixed in with a struggle. Battles felt like there was a lot at stake for the characters. Anxiety and mystery are its key themes.
Return of the Jedi, the battles feel very bombastic and much more light hearted. Not just due to the Ewoks either. The battles on Endor feel like a waste of time when it comes to plot development and storytelling. Neat action scenes, but they're pretty empty as a whole.
The best thing about ROTJ is Luke, Palpatine, and Vader all in the same room and the context behind the drama. Those parts of the movie rival everything great from The Empire Strikes Back.
If the movie worked around that conflict, of sith trying to convert Jedi, more than Endor and space battling, it would be viewed a lot more favorably.
I think the hate it gets is weird.