'Creative differences' are part and parcel of the movie business. Everybody's personal preferences are entirely subjective and in a medium as frequently divisive as cinema, there are plenty of disagreements from the studio boardroom all the way to the ride home from the theatre.
A movie set has so many moving parts that it becomes increasingly difficult to keep things running smoothly at all times, especially when a big budget is involved. Sometimes, a director doesn't realize they aren't up to the task until the cameras are already rolling and the people behind the scenes take notice.
In other instances, the filmmakers are constantly butting heads with the real people in charge (usually the producers) about the creative direction of a movie until their position ultimately becomes untenable, as the recent bombshell surrounding Phil Lord and Christopher Miller will attest.
McG and Uwe Boll have never been kicked off of their own set and have yet to make a decent movie between them so it clearly has nothing to do with talent. Getting fired from a movie doesn't make you a bad director, just an unfortunate one caught in the wrong project at the wrong time.
10. Phil Lord and Christopher Miller - Han Solo
After the apparent removal of Gareth Edwards for reshoots and post-production on Rogue One and now the highly-publicized firing of Phil Lord and Christopher Miller with just three weeks of shooting left on the untitled Han Solo movie, it has become definitively clear that Kathleen Kennedy is in charge of the Star Wars household.
Kennedy reportedly disagreed with the duo's loose, improvisational and comedy-heavy style of filming, while the directing duo felt that based on their previous track record that was the movie they had been hired to make. Feeling that the project had veered too far of-course, Lord and Miller were removed from the project, with Ron Howard drafted in several days later.
For some, the Han Solo movie was exciting precisely because Lord and Miller were making it and while Howard is no doubt a safe pair of hands, the veteran director has a short time period to turn the project into whatever it is Lucasfilm wants it to be with so much footage already in the can, not to mention the various negative rumors coming from the set on an almost daily basis.