TLDR: VERY poorly animated, bad use of cameos, wasted potential plot points, cringy characters, cheesy fights, and ATROCIOUS writing.
World Heroes Mission suffers from a lot of things. Generally the writing is weak and illogical, particularly with heroes not having a system of international law to study and adhere to, as if ANY of the 3 at Endeavors agency would neglect that. The humor was juvenile and way too simple. Probably the critical factor is that, in comparison to the past two films, WHM does not draw upon the events of the corresponding season or the manga. Where heroes rising does this almost flawlessly, WHM neglects the meta liberation army and similar groups, all of which would NOT have been passive against a genocidal anti-quirk cult. The movie would have benefited from such a plot point, as well as from better contributions from heroes across the world.
Rather than do this, the movie shoehorns all might, a single frame cameo of godzilla, and silly hero ideas such as the 2d egyption drawing, not to mention that Rody was a poorly integrated side character and the roadtrip on the lam was FAR too long. Even main characters like Totoroki and Bakugo are underutilized and the fights are absurdly cheesy. 2d and 3d animation are poorly integrated to the point of being distracting, the final fight ended up being a sub par recreation of the all might-Nomu fight that went more against the lore than anything from past movies and filler, and if the movie didn't spend so much time trying to mimic spider man then maybe they could have animated scenes so that the frame rate didn't sink to windows 95 3d game level. The only decent fight is Bakugo's end film battle.
I was very disappointed with this film and it was a waste of potential, because it COULD have easily been the best mha movie but was just bad.