The Justice League movie didn't swing the DCEU around, much as Warner Bros. had hoped it would. Instead, the league sank into rotten territory on Rotten Tomatoes, with slipshod editing being among critics' top complaints with the film.
Due to Zack Snyder leaving midway through the film's development and Joss Whedon subsequently taking over, there are two distinct tones in Justice League: Snyder's grim realism and Whedon's campy optimism. While that's not an easily remediable tonal disparity, it is something the film's final cut should've done a much better job of rectifying. In its theatrical release, the movie was panned for coming off as an inferior Avengers-wannabe due to its meshing of past DCEU tones and lighter Marvel-esque elements, neither of which played well together thanks to cuts that awkwardly inserted jokes and serious moments without any rhythm.
Worse yet, the haphazard editing led to important exposition being cut from the film in order for the studio to achieve its desired two-hour runtime. As such, entire scenes were rendered borderline pointless due to being incomprehensible. A perfect example of this is the entire Atlantis sequence, which is edited in such a way that no character besides Steppenwolf receives even a shred of clear character motivation.
These are some of the biggest reasons fans were (and are) petitioning for a purely Zack Snyder cut of the film. Sadly, even that rumored version likely wouldn't be able to remedy Justice League's largest editing flaw—namely, mustachioed Superman. The solution to remove actor Henry Cavill's mustache, which only existed when he returned for re-shoots, was ham-fisted. The options it seems were to edit all previously filmed appearances out of the final or to add a beard to make him look weathered upon his return—options which were apparently untenable. Perhaps the editing isn't the only thing to blame for this debacle.