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.
I think that you have the wrong perception of how the EU depicted the Light Side and the Dark Side of the Force. So, I would like to make some corrections.
In the EU, the word “Grey Jedi” was not used to describe Force users who use both sides of the Force at the same time. On the contrary, it was used to describe those members of the Jedi Order who, despite not using the Dark Side, at the same time don’t always followed the rules, and don’t always listened to the will of the Council. The word was used to describe the more rebellious members of the Jedi Order, and was never associated with the Dark Side of the Force. Never. And, it’s not like the word was often used, anyway. In fact, it was only used a couple of times through the entire EU. Furthermore, even though the Je’daii Order existed, and even though they tried to create a literal balance between the Light Side and the Dark Side, it was strongly implied that their ways were wrong, precisely because you can’t balance the Dark Side with the Light Side. In fact, the Je’daii Order fell apart.
When the EU was still Cannon, the Light Side was always considered as the true balance and the true natural state of the Force, and it was always explained that the Dark Side corrupts everything. Yes, some attitudes of the Jedi Order (and some individual Jedi) were criticized, especially during the Prequel era, but the Light Side in itself was never criticized, and being overly-attached was always seen as something negative that could lead to the Dark Side, even in Luke’s Jedi Order. Hell, in the Darth Plagueis novel, it was literally explained that Anakin was created by the Force as an automatic reaction, in order to stop the Sith from corrupting the Force itself. So, it was always implied that the Sith should be destroyed, because they corrupt the Force and create unnatural stuff, indeed.
It’s the New Canon the one that introduced the concept of “equal balance between the Light Side and the Dark Side”, not the EU. The EU always depicted the Dark Side as a cancer, and in all the stories that featured Force users who tried to balance the two sides, it was always implied that they were wrong. The problem is, though, that a lot of people on the internet don’t understand this, because they don’t actually experience the EU first hand. They just learn stuff from other people telling them, and from YouTubers who understand nothing about the EU themselves. And, because of this, we have people using the word “Grey Jedi” in the wrong way, and who unironically think that the Je’daii Order was correct, when they never managed to maintain the balance they wanted to maintain, precisely because such balance doesn’t exist.
Finally, this has nothing to do with the EU, but I wanted to point it out anyway. The Mortis arc in The Clone Wars was created by Lucas, not Filoni.
Edit: When I talk about the EU, I’m not including TCW into it. Yes, the first 6 seasons of the show are technically part of both Legends and Canon, but I prefer not to include TCW into the Legends timeline when discussing it. I see the show as being part of Disney’s New Canon only. The EU version of the Clone Wars is the one that was depicted in the Clone Wars Multimedia Project, while the Canon version is the one that was depicted in TCW. I prefer to maintain them as separate.