Note: This is my opinion and everything expressed here are fully My Two Cents .
I want to first start off by saying congratulations to Zack Snyder for a successful movie despite the personal and professional issues. You can read my Zack Snyder's Justice League review for details but this opinion piece isn't about him. It's about his colleague Joss Whedon and I do promise to be as nice as possible because this isn't a smear campaign.
While filming Justice League March 2017, Zack Snyder learned his 20 year old daughter Autumn committed suicide. He continued to work for two more months but the grief was overwhelming to him so he left the project. For someone who understands grief, I understand the need to step down and reset. Both Zack and his wife Deborah Snyder were both on the project and both agreed to leave the project and allow someone else to finish it. Enter: Director Joss Whedon, better known for projects like Buffy The Vampire Slayer (TV), Much Ado About Nothing (Movie) and Angel (TV) and most famously for directing the first two Avengers movies (Avengers; Avengers: Age Of Ultron). That would have been an idea except it seemed like trouble from the start.
At first I assumed being Zack Snyder's friend, Joss Whedon would do right by him and finish his vision especially with everything going on with his family. Joss' only job was to see the crew through post production and reshoots. That turned into major nightmares from the crew. Without going into detail, there were many complaints. One of the most famous complaints came from Ray Fisher (Cyborg). He claimed "gross abuse" on the set. He was accused of being unprofessional. When word of this got out, members of the former Buffy The Vampire Slayer crew and others rallied around Ray Fisher and talked about their own tales of abuse.
I personally think Joss Whedon personally sabotaged the first Justice League project whether he knew it or not. I believe he did. Even though he wasn't involved in Avengers: Infinity War my feeling was he didn't want Justice League to outshine the next Avengers movie which came out a month later than Justice League. He took what was to be a powerful movie (for those who's seen Zack Snyder's vision) and turned it into a cartoon like light hearted adventure/comedy. It definitely wasn't up to par with Infinity War.
This not by far a critique of his work because the projects he works on are great. In fact stellar. I just don't believe he should have been the one who took over the Justice League project.