The hate of them, when you boil it down, stems from change.
For twenty years, fans could decide what The Force was and what it meant and how it worked. We could debate it, ponder to our heart's content, and at the end of the day still not know who was "right".
When tPM came out, that changed. The Force was further defined, and done in a way very different from what a lot of fans wanted it to be. Quality issues aside, many of the issues we older fans have with midichlorians, the Jedi purge, the Clone Wars, all of it stems from decades of imagining being swept away.
You have a similar issue now with the Cannon/Legends divergence. Suddenly many characters, plots, or ideas that just WERE Star Wars to many fans have become, to an extent, wrong. It's the same let down: something you devoted emotional and intellectual energy to is just rendered obsolete. Gone, like suddenly being dumped.