I'll defend the original FMA from being bad. The twist of homunculi being created after failed transmutation attempts led to some of the grimiest scenes you'll ever see in shounen. The only way to kill them was to dig up the remains of their bodies, so imagine what that means Ed and Al have to do to beat a particular homunculi. Wasn't as comprehensive as Brotherhood but still hits hard.
Why is the manga of anything better to everyone ?
It's the story as intended by the author. An anime may deviate from the source material into a worse conclusion or make some stylistic changes that don't match the tone. I've seen an anime adaptation that was so different from the manga that the author got mad and the director wound up changing mid production (His and Her Circumstances; Hideaki Anno resigned after episode 18). No different from a movie adaptation of a novel that chooses the wrong actors to represent the characters and leaves out details because of a director's preference and/or time constraints.
Manga is much less time consuming to read. One chapter takes 5-10 minutes in most cases. Anime episodes have to be 22+ minutes long and some will use that entire time to cover a single chapter intentionally stalling on scenes just to reach that time frame. This is why One Piece sucks right now.
The large majority of anime adaptations are advertisements to buy the manga, light novel. They don't add anything of note to the source material. Stuff like HxH 2011, Brotherhood, JoJo are the exception to that rule.
and even if a manga adapts a part of an anime perfectly, i.e Berserk (1999) and Hajime no Ippo, if it doesn't cover the entire story then it's inherently the inferior product.