You can probably glean this from all my other posts across the site, but the most damaging parts of the prequel/EU/Filoni system involve the portrayal of the Jedi, dark Jedi, Sith, the Force, and the dark side.
“Gray Jedi”, “balance in the Force”, “using both sides of the Force”, “J’edaii” or whatever they’re called (also “Jensaarai” lol) the “light side” and “dark side” being equal and neither good or evil, Mortis BS, etc.

There’s exactly one YouTube channel (Geetsly’s) I’ve found that has a correct perspective on all of this, and every video that talks about it is swamped with the most cringey fanfic-tier comments about how both sides are wrong and UMM ACTUALLY THE JJ’EEDAIII (whose name those stupid Jedi freaks STOLE) WERE PERFECTLY BALANCED and if you went too far to one side they would banish you to the opposite moon until you got BALANCED again. Revan and Anakin and my OC Darklight Lightdarker can use the LightDark DarkLight GrayDark GrayLight LightLight DarkDark Side with no ill effects because they’re emotionally BALANCED.

This started with the prequels but EU writers and Dave Filoni made it much worse by making it canon depending on the story.