Ok, well, I'm gonna try and give a fair read, but I am insanely biased here. You can find the old lores online if you google for them to make your own decision. But here goes:
Neither lores are really all that great in their entirety. Neither are going to be winning any Hugo Awards any time soon. There were good parts and bad parts to both.
I'll start with the old, what was worse about it? Targon itself. Originally Targon was called Mount Gargantuan home of the Stanpar (literally an anagram of Spartan) who were the greatest soldiers ever. And Pantheon was the greatest soldier of the greatest soldiers ever. And that's where it sat for awhile. Now we can learn a bit about the culture from Pantheon's quotes, and how they viewed warfare, but it was really what you would expect. A far more brutal take on honor, and love of violence for violence sake, arrogant as all hell. Your basic Hollywoodized Spartan.
Then IronStylus came and added in Leona and Diana. And here's where it started to get interesting. The Stanpar were changed to the Rakkor Mount Gargantuan was changed to Targon and I think most people were happier for it. Those were really silly names. Leona was given the story of how she was once a Rakkor, was friends with the young Pantheon but rebelled against their brutal ways (they had a thing called 'the Rite of Kor' where they murder each other, obviously inspired by the Krypteia but it was very dumb and makes no sense from a cultural survival standpoint). She was in turn to be executed, but was saved by being chosen from by the Sun who was presented as an actual god. Despite granting her powers, the Sun took a bit of a back seat to Leona allowing her to do her own thing, and she trained with the Solari to one day lead Targon.
Then Diana was released and we saw that maybe the Solari weren't as good as they at first seemed. We see this outcast girl who like Leona rebelled against what her teachers were saying. So she out on her own went looking for clues of the power of the Moon and found them. Heading back to the Solari elders she excited tells them of the old temple she found, only to have them condemn her to death. At that time the Moon granted her power, and she used that to murder the Solari elders. And continued to wield it like a cudgel against all those who would deny her power.
Now from that, plus Leona's character in her League Judgment (a thing where they analyze a champions inner demons and character for them to join the League of Legends) revealed that Leona was a very good person, filled with a bit of self doubt if she was worthy of the Sun's power, plus confusion if she would one day piss the Sun off and lose that power, and if she would grow too dependent upon it. She also wanted to change her home. Diana, on the other hand was filled with hatred over everyone who tormented her.
Added onto that was IronStylus's own hints that Leona and Diana were once friends, possibly more. And their paths will lead to unhappiness between them all.
So we were given a naturally good and kind character (Leona) who was presented in the wrong. The Solari she defended were actively attempting to stomp out Diana's truth. Meanwhile the one who was right (Diana) was a ruthless murderer. While all of Targon would hang in the balance of what happens (where I assumed Pantheon would come back into the story, representing the normal people of the mountain).
The new lore took a lot of what was written above, but warped it in ways that were pretty strange. Leona wasn't really a nice person anymore, she was the one who was day dreaming about being a petty executioner before Diana had even done anything wrong. Diana just sort of tries to find peace after going on an all out murder spree in the temple which also jars with her in game dialogue. Then there's Pantheon, who, actually in the first part of his lore seemed to have a pretty great expansion of his character, then the biggest problem that plagued all three of the lores suddenly happens. Space aliens show up.
And suddenly the choices of the characters aren't really theirs anymore. Leona, who was once just a good person turned into a vicious person only to have space aliens show up and suddenly she's good again. Atreus had a fantastic story about trying to defend his homeland from foreign invaders while setting the stage for potential conflict between two tribes; the Solari and the Rakkor over how their mountain should be defended. Then suddenly space aliens show up! And he randomly becomes the biggest tool, ever.
And it wasn't even set up particularly subtly, throughout Sol's lore the Targonian space spirit things are just massive unlikable assholes. And what glimpses we see of the spirits that are taking a far more active role in controlling their corresponding meat suits don't make them seem anything but jackasses as well. Really the only one to come out of it looking well was Taric, who was never a Targonian champion before anyway.
So that's it. While, from a technical perspective I can say, yeah, the new lore is probably better written. The actual narrative path taken was just so out of left field, ignored the conflict the old fans were all hoping to see, and instead shoved space aliens into a lore that really had no need for them. That's why some of us really dislike the new lore.
