It’s pretty common knowledge that most anime is an adaptation of something. Manga, light novels, visual novels and even mobile games all provide source material for anime. There are a few original anime, but they’re vastly outnumbered by all the adaptations produced every season. As with any adaptation, there are plenty of people who are fans of the original material and complain if the anime changes things and that following the original work is always better. The problem with this mindset is that there’s nothing inherently better about following the source material. All that means is that the anime will have the exact same strengths and weaknesses as the original, so if the original did something badly, so will the adaptation. That said, in my experience, adaptations that stick closer to the source material tend to be better, but not for the reasons people normally give.
One of the most famous anime adaptations that diverged from the manga is the 2003 Fullmeatal Alchemist. The manga was still years away from completion at the time, so the anime had no choice but to go into original content once it caught up with the manga. A lot of people have said that the anime original material wasn’t as good as the parts that were closer to the manga, and to an extent they’re right. I’ve said before that the ending to FMA 2003 wasn’t as good as the rest, but some of the other changes the anime made were actually beneficial. Spoilers for FMA 2003 and Brotherhood ahead. Skip the next sentence if you haven’t seen them. Maes Hughes had relatively little screentime in the manga and in Brotherhood, but the 2003 anime gave him a lot more attention and development, so his death had far more impact there compared to the source material. Similarly, Lust’s characterization was vastly better in the 2003 adaptation.
The Ghost in the Shell franchise is another example of an anime that diverged from the source material but turned out well. Stand Alone Complex took virtually nothing from the manga beyond the setting and characters, but nobody ever criticizes it for making changes. The various anime adaptations of GITS are probably more well known and more highly regarded than the manga at this point.
Like I said before, though, anime original content is often inferior to the more faithful parts. The endings to Soul Eater, Claymore, Fullmetal Alchemist and Blue Exorcist all diverged from the manga and suffered for it. I don’t think this is because anime only=bad, but because writing anime original content for an otherwise faithful adaptation places an extra burden on the writers. Instead of just having to write a good story, they have to write a good story with someone else’s characters and fit it into a world that someone else created while following all the rules and prior characterization that was established. If that sounds hard, it’s because it is.
This problem isn’t even exclusive to adaptations that can have this problem. Psycho-Pass 2 was a distinct step down from the first season, mostly because it was written by Tow Ubukata instead of Gen Urobuchi. Ubukata isn’t a bad writer at all and did good work with Mardock Scramble and Ghost in the Shell: Arise, but he clearly struggled with keeping Psycho-Pass 2 consistent with season 1. Inconsistencies began to crop up near the ending, making it obvious that there was a different writer with a different style and different ideas. By the end, it was clear that Ubukata’s interpretation of Urobuchi’s world and themes wasn’t how Urobuchi intended it, and the inconsistencies with season 1 are where a lot of the issues came up. Psycho-Pass 2 wasn’t an adaptation of anything, but its problems were exactly the same as many adaptations that diverge too much from the source material.
I’m not trying to say we shouldn’t criticize anime original content because it’s hard for the writer or that all changes are bad. My point here is just that we shouldn’t criticize an adaptation just because it diverges from the source material since that doesn’t automatically make it bad, even if changes frequently don’t work out. It’s better to look at an adaptation by itself and judge if something in it is good or bad on its own merits.