If WB were as dissatisfied with Snyder as certain groups of people claim, they could have fired him outright at any point during the film's production save for the final month or so of shooting.
The very fact that they didn't is in itself a death knell to the idea that WB was in fact dissatisfied with him, but certain people clearly dont actually care about what is factual and would rather cling to whatever narrative best lines up with their own opinions.
I don't think Zack Snyder was the problem for this movie. I believe that the WB suits and Joss Whedon were the real problems . . . at least for me. They chopped it up a bit too much. In fact, the movie reminds me of "Age of Ultron" in many respects - with the exception of the running time and that the Marvel film only had one director.
I also don't understand this attitude about the "darker aspects" of Snyder's DCEU films. I mean . . . so what? Even the MCU had its more grim movies.