What the vast majority of lengthy, mixed reviews of FMAB are trying to get at is simply that it's a shonen. Due to the show's reputation, they attempt to describe its flaws on unique terms, at times plunging into an absurd level of nitpicking to do so: but the essence is always the same. This places discussion regarding its faults in an awkward position. On one hand, a banal exercise in naming generic and undesirable characteristics of the entire shonen umbrella genre doesn't feel worthy of such a highly distinguished series in terms of reception. On the other hand, it is necessary if we are to actually say anything about it.
There’s nothing inherently wrong with shonen, but there are facts regarding it that exist by virtue of the group it aims to appeal to. Here it manifests as on one-trope characters, juvenile humour, and fight scenes which should more often than not be forgotten as soon as they finish. The soundtrack might as well not be there, and the way in which episodes are sequenced achieves the spectacular shonen duality of being entirely predictable while at the same time coming off as decided by a random number generator. Social and philosophical "themes" are present in the same way as in Christopher Nolan films in that they are more so shower thoughts than developed concepts; the show enjoys the aesthetic of being complex for its genre without any of the meat. It seems to me that this dieselpunk, "thoughtful" aesthetic is really what makes it most appealing to its fans along with their love for the source material. Serious dialogue often sounds like sentence prompts your English teacher might have warned you not to use in your creative writing - think "and then he woke up". And they might have for once agreed with the students that the curtains really aren't red for some highly sophisticated reason.
Look, the show is fun and a-okay, and I urge anybody who is a dedicated hater when it comes to popular shonen but makes an exception for FMAB to do some self-reflection. It’s digestible, very non-offensive, and a safe choice to watch with friends when you want a good time. There’s also little to dig into, but plenty of people do not care about that. It’s fine to find a show entertaining for unsubstantial reasons. It’s also fine to dislike a show for unsubstantial reasons, as long as we don’t selectively enforce when repetitive humour and shallow characters are a deal-breaker. Did I mention Ed doesn’t like being called short?