The directors behind Infinity War and The Avengers: Endgame happen to think Superman is a difficult character.
In an interview with Business Insider, the Russo brothers were asked which comic book or superhero do you think is the hardest to bring to the screen.
"The more powerful a character is, the more difficult to deal with that character on a narrative level," Anthony Russo said. "As storytellers, and the way we explore characters, we always look for vulnerabilities in characters because that's where characters become interesting. They're superficially interesting in their strength, but they get much more depth when you find where they don't have that kind of strength. In general, the more powerful a character is, the more tricky that is."
The interviewer then specifically mentions Superman.
"Yeah, exactly," Anthony Russo agreed.
"He's a very difficult character," Joe Russo added. "You have to find an emotional flaw or weakness in the character in order to make them vulnerable."
"That's why Vision fell in love," Anthony Russo said.
The Russos did happen to do a pretty good job with arguably one of the most powerful characters of all time with Thanos in The Avengers: Infinity War, where we also see they added emotion to bring the Great Titan down a level or two, which did work.
As this site is "Cosmic," writers stating they have difficulties dealing with powerful characters is something I have heard time and time again. A lot of writers seem to find it easier to write the "street level" characters such as Batman, Spider-Man or Daredevil - characters which I find boring - but some writers do manage to tackle the cosmic heavyweights and do a good job such as Jim Starlin, Greg Pak and Dan Abnett (off the top of my head).
Regarding Superman, I thought Zack Snyder did a decent job in Man of Steel as he provided characters as powerful or more powerful for Superman to take on. Superman also has a supporting cast with his family and Lois Lane, in addition to being Clark Kent, that can humanize him a bit.
Marvel's Phase 4 will be interesting as it is said to be going cosmic in a big way. The cosmic characters have always been more powerful, so it will be interesting to see how Marvel Studios deals with characters such as the Eternals and even possibly Nova, Silver Surfer, Galactus and more.