In my experience the people who hate on Rebels are the same ones who hate on TFA and haven't shut up about how awful the prequels were for the last fifteen years but when asked why either can't give a real reason or name an issue that was also present in the OT. They are the elitests who don't think anyone outside their generation should be allowed to enjoy Star Wars and thus anything that doesn't bend over backwards to pander to them is garbage.
They were simply bad movies, though Revenge of the Sith was OK and the Original Trilogy (even without later fiddling) were... good. Not brilliant, just good. The plot was not so important, it was the characters that took centre stage. And that is why TFA ranks amongst them, maybe even better.
Part of what makes movies go from good to brilliant is pure accident and serendipity. The opening barracks scene of Full Metal Jacket was entirely AD LIB BY A REAL DRILL SERGEANT TERRORISING THE ACTORS. Ford and Fisher had a real-life troubled love affair. The SFX department convinced GL not to do 50s shiny rocket ships, which would have looked awful. The Falcon was redesigned at the very last second. etc.
Edited by Lampyridae