To an extent, yes. Shonen Jump (and all manga publishers, for that matter) operate pretty much on the ad populum fallacy of appealing to the majority rather than focusing on quality, leading to very few manga that can remain entirely coherent under such a schedule. As you brought up FMA, Arakawa's decision to let the original anime go its own way whilst she took more time to examine and develop her characters shows the restrictive nature of the manga industry.
That's not to say many series can't work around this nature. Whilst DB had plenty of flaws, it was able to remain a lot more coherent than a lot of other Jump series of similar length thanks to Toriyama's improvisational nature and only choosing to flesh out plot points that he either intended to make relevant or if he could use them for later inspirations (eg. the many things pointing towards Goku's alien physiology despite him not having thought of it until the end of the Piccolo Daimao Arc). JoJo's Bizarre Adventure also found a way to avoid huge pitfalls despite its length through its nature of each part having a new protagonist and setting to avoid continuity having to become too meticulous. By comparison, the series that try to have a big plan or try to add too much on in between the planned beginning and end ultimately tend to shoot themselves in the foot. One Piece has become a lot less consistent since the timeskip with how Busoshoku Haki and its physical colouration now just being a thing despite not appearing once amongst the veterans at Marineford and the constantly widening gap in power between certain characters and the top tiers which make a lot of things in Part 1 very shaky to say the least (Luffy being treat as a danger to the Shichibukai for beating Lucci, Mihawk finding such interest in Zoro, etc.) and it's quite telling when he initially only planned for the story to go on 5 years.
When it comes to most manga, be it in Jump or elsewhere, the best route to maintaining a consistent story seems to be having a simplistic set up for your story's start and fully exploring whatever ideas you have for its grand scale once you've gotten past the initial phase of publication. There's also, of course, the option of only bringing your work up to your planned ending and not bending to milking it further. Whilst Assassination Classroom was a mediocre at best series, I've got to give the mangaka some respect for ending the series on his terms rather than extending it through huge contrivances.
Fantastische Hure said:
full-metal-alchemist didn't run in weekly-shonen-jump i think and that's 1 of the most well-received series i think.
Correct. It ran in Monthly Shonen Gangan
