I never read the books, but I remember that whenever I thought a plot development was REALLY interesting, someone inevitably complained about it being a drastic change from the books lol. I suspect the showrunners *wanted* to deviate from there in some major ways (i.e., it was clear the character that interested them most was Alec, for example), but didn't quite have the guts to fully depart from them. The results were mixed lol.

I'm only vaguely familiar with Fairy Tail('s title). Ig the equivalent for me would be ATLA or HP; I even broke my lurker ways back then to comment on fic forums and all lol, but I rarely think about them these days, save the odd ATLA post that crosses my dash lol.