Having read HIBC I was genuinely expecting it to be an unbearable experience...it's not. No, really, don't let then pearl-clutching and hand-wringing fool you, it's not. Does it shit on characterization? A little. Really, I have to say that, never having been a huge fan of ANY Fire Nation character (but Ty Lee, who is my wifey) I really can't honestly say that they're ALL out of character; Zuko is still a creep with good publicity (and abs) and Mai is still an unironic glorification of everything that made Daria unbearable. Aang and Toph are both still sweet, kind individuals, Katara is still an outright hero, Sokka is largely still a horndog lovable asshole, the biggest changes are Azula (COMPLETE character rewrite) and the actions of Mai to some extent but then again all the Fire Nation folks in the series were just Nazi Party: Red China District. Really, would you put it past ANY of those kooks to kill a baby? Before you answer remember that Zuko burns a town down to find the Avatar once, and he's basically the HERO OF THE SERIES.
Except Ty Lee, she's the breast of them...best of them!
Really I think the reason I enjoyed it more than some was, I have no notion of Zuko being heroic or Mai being a sweet innocent victim, so either one doing something horrible like adultery or murder bothers me not one bit. Also, I love soap operas and the whole thing is so lavish and so dramatic and fantastical it either has to be a parody or straight rip of of soap operas, and the signs are all there.
Love child? Check.
Illicit affair? Check.
Tragic miscarriage? Check.
Forbidden PASSIONS? Check dat.
Evil mistress? Check.
Nurse my beloved back to Health? Check it before you wreck it.
Cheesy dialogue? GEE YA THINK!
May as well call it Dallas: How I Became J.R's! In fact the exact same plot played out on that show, TWICE, in the reboot and the original series. It's been done three times I know of on One Life To Live, and I lost count on General Hospital. If you like soapy drama it won't bother you more than any other canon-annihilating fanfic. There are far worse, far far worse, so give it a chance...and a rest.