Yeah, Lucasfilm has been messy, but it's really hard to go from a studio making 0 films to one making multiple ones in the same universe. Marvel was really messy in the beginning too.
In April 2006, they announced their initial slate as a studio:
Iron Man, directed by Jon Favreau
Captain America, written by David Self
Ant-Man, written and directed by Edgar Wright;
Nick Fury, written by Andrew Marlow
Thor, written by Mark Protosevich
The Incredible Hulk, written by Zak Penn
Iron Man did end up being released on time and directed by Jon Favreau, though finding writers willing to work on it was quite hard.
David Self did not end up writing Captain America.
Edgar Wright did not write or direct Ant-Man and it wouldn't be released for 9 years.
Nick Fury never happened at all.
Thor was written by Mark Protosevich, but not before he was replaced and then came back.
The Incredible Hulk did end up being written by Zak Pen, but actor Edward Norton rewrote much of it, some during filming.
In 2008, the same weekend Iron Man came out,
Marvel gave an update on their movies.
Iron Man 2 would come out on April 30th, 2010 and would feature the first appearance of Thor.
Thor would be directed by Matthew Vaughn and would come out on June 4th, 2010.
The First Avenger: Captain America would release on May 6th, 2011.
The Avengers would release in July 2011.
Ant-Man was still being developed by Edgar Wright.
Iron Man 2 did not end up releasing on April 30th, though to be fair it was only bumped a week. It also did not feature Thor and recast James Rhodes.
Emily Blunt was originally cast as Black Widow but had to back out after Fox exercised an option to have her make Gulliver's Travels.
Matthew Vaughn rewrote Thor, then left the project. Marvel flirted with Guillermo Del Toro to replace him, but Del Toro turned it down to work on The Hobbit. The movie would end up being delayed by a year.
Captain America emerged relatively unscathed. It got delayed by a couple of months and got a small title tweak.
The Avengers was delayed a year after Joss Whedon was hired and threw out the existing script telling Marvel they had "nothing." The Hulk was also recast.
Phase 2 was better but still a little messy. Captain America: The Winter Soldier and Guardians of the Galaxy went fine and Ant-Man had no additional troubles. But Thor: The Dark World and Avengers: Age of Ultron still had issues.
Thor: The Dark World was originally set to be written by Don Payne and directed by Brian Kirk and release on July 26th, 2013. Kirk backed out and Patty Jenkins was hired and the movie was delayed until November 2013. Then two months after she was hired, Jenkins left. The part of Fandral was recast because the original actor Josh Dallas had a conflict. So Zachary Levi, who would have played the part in the first Thor if he hadn't had a conflict with his show Chuck, took the role.
Joss Whedon had a miserable time on Avengers: Age of Ultron, chafing under all the studio-mandate bits he had to include. Captain Marvel was set to appear in the movie at one point, but it took so long to cast her that she was eventually cut. A shot at the end of the movie was meant to include her but was changed to feature Scarlett Witch instead. Speaking of which, Whedon had to redo the backstory of Scarlett Witch and her brother Quicksilver since they were mutants in the comics and 20th Century Fox owned the film rights to mutants.
