‘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.