my issue with Embers is that though it's excellently written, I felt it got too caught up in its own mythology and kind of left some of the original ATLA elements behind... I don't mind OCs, and Embers has a lot of well-written OCs, but I kind of wished for a bit more focus on some of the ATLA characters.
Also too much of the characters' issues were explained as personality traits of the nation / culture / spiritual whatever, which edged out their personalities a bit, imo.
/my two cents