Dang
there are three main reasons i say this.
one) zuko is never actually challenged by the narrative and made to question his imperialistic, racist, and xenophobic beliefs about the alleged inferiority of other nations, people, and bending.
you can say, "but he realizes he sucks during zuko alone!"
but i have to ask you: does he ever actually question the fire nation's values? or does he just realize he's been mean and scary?
his redemption arc is largely made into a personal issue of kindness instead of a social issue of him needing to make amends for both enforcing and benefitting from the fire nation's imperialism.
furthermore, zuko's redemption is boiled down to a moment in which he chooses a family member who is not abusive to him over a family member who is abusive.
two) zuko's redemption arc relies on audience bias. you like zuko because of how sympathetic the narrative is to him even when he's being a racist imperialist, and thus you want zuko to redeem himself. you root for him to be part of the gaang because of all of these moments of sympathy given to him by the narrative and these little moments he has with them that tease at his potential goodness.
zuko absolutely gets to the point where he starts to make better choices in the narrative and wants to do better. i'm not saying he doesn't.
but does he ever textually earn the gaang's trust and make amends with them? you might say, "well, yeah, he goes on life-changing field trips!"
think about it, though. how quickly does the gaang decide they like zuko fully now in book 3, despite him having hunted them and tried to kill them repeatedly throughout books 1 and 2? what does zuko actually do besides take a few of them on "life-changing field trips?" how does the gaang trust zuko so much after a few weeks of him being one of them and months of him being their enemy that they don't even think it's suspicious or weird or scary when he starts firebending aggressively at aang?
the answer is that bryke took too long getting to zuko actually starting his redemption arc and relied so much on audience bias that they decided the gaang had to forgive zuko and trust him that easily, regardless of it made sense.
three) zuko's choice of his uncle over his father is a matter of filial piety: should he abide by it by paying respect and obedience to his father or break it by choosing to go against his father? however, it is never portrayed through this lens. the choice is whitewashed and westernized for the consumption of a white, western audience. it gives no nuance to what it means to be raised with filial piety as an unquestionable staple of your culture and go against it.
this last one is racist, if you were wondering. because atla is racist.