Is well written the same as good?
The question probably sounds rather tautological and obvious but it's actually the main issue I have with the fanfic. As it stands, it is well written. The story is strong, the plot is complex but internally consistent and seems mostly free of twists pulled out of bottoms. The characterisation in the story is pretty consistent. HOWEVER. (And this is capitalised for a reason.) However... the characterisation in the story is internally consistent, but very much less so with the characters as presented in the show. The names are the same, they look pretty similar... but for me? That's really about it. Zuko in this story is treated as if he's undergone his entire canon character development, and more besides, before the time of Ba Sing Se, which canonically was when a great deal of said development actually took place. The flaws that remain are of the 'informed' variety that tend to somehow conveniently work in his favour as much as against it. Other characters (both canonical and original) are presented as being more 'realistic' versions, deconstructions almost. Which is laudable, except that everything before the story starts is meant to be the same as canon, and characterisations and personalities that I never recall seeing in the series suddenly pop up in the Embers characters. Cultural attributes and values shape the characters... attributes and values that never appeared in the show. The past of the Avatar world is made up pretty much entirely of old cloth and whatever new idea the author wants to push right now. Some of the above can be explained (or excused, if that's the term preferred) in that the canon backgrounds had not been explored in any great depth in the show. With that said, it takes a very certain type of mind to take the pacifist, Buddhist-styled Air Nomads and somehow reach the conclusion that this comes from the end result of them once being Mongol-like conquerors and imperialists. Other tribes get similar treatment. The other issue that WILL arise is that the author has characters they like, and those they don't, and even a cursory reading will make it extremely obvious which is which. At some points it can be hard not to feel that certain characters literally cannot do anything right. Whatever they do, the plot will unfold in exactly the way as to make them wrong to do it. If you like the characters the author doesn't, or don't much care for the ones they do, the seven deadly words may well be uttered. In summary, I would have to say that if this was an entirely original story with completely new characters and a setting the author had made up for themselves I would not hesitate to completely recommend it. But as a fanfic of Avatar, even an AU one? I would still probably recommend you try it, with reservations - but do not go in expecting this to be the same story and the same characters you watched in the show itself.
Embers is...well, it's certainly an interesting story. It started out life as an Avatar: The Last Airbender fanfic, but by now has diverged significantly in tone from its founding source. If you like elaborate world-building, rules about spirits, the development of culture when concepts such as justice and family are a spiritually binding force on the people, and discussions of morality, it's well worth a go. That said, the characterisation is polarizing — some people love it, believing that the characters are deepened by Vathara's use of real-world psychology in the same way the setting is improved by her use of real-world myths and geography. Others, however, find that her interpretation of the characters clashes with the sense of them they get from the show, and disagree with Vathara's take on morality and responsibility, which is a great deal harsher than the show's. Fans of Aang and Katara may want to give this one a miss; Vathara writes very well, but the flip side of that is that when she dislikes a character, it hits home hard.