I've understood the argument that characters on a bigger scale i.e. Superman, Thor, Silver Surfer, Galactus, Martian Manhunter, etc, are harder to write. The bar has to be raised to their level, and we're not nearly at that level.

Still, the limit is on your imagination. If you can dream it, it could happen to any of these characters.

More than knowing how much the character can bench press or how quickly they can circle the globe, are things like, new forms of life and the threats to it. Being stronger doesn't necessarily mean you're able to out think a galactic super mind, or being invulnerable to an undiscovered virus, or being caught in a galactic storm and battered senseless and left adrift.

Are those things we couldn't identify with ?

How to choose where to be ? What's the greatest crisis ? What's more important ? Stopping a Super-Villain Of The Day or a monsoon with thousands of lives on the line ? What about the government asking for help ? Which government ? What organization ? Do you do charity events ? In doing that, does it endanger innocents there to see you and contribute ? Do you protect them ? How can you ensure their safety ?

What if someone set you up with an evil double ? What if it was something you didn't want to do but they wanted to show you doing it ? Do you have a robot squad to help ? Artificial intelligence ? Could they go rogue ? What if saving Earth meant balancing our lives against a parallel dimension ? Would their super characters be heroes or villains ?

There's all kinds of stuff that's possible, but it may not be possible with corporately owned characters. That's why some of the "knock-off" characters, like Mr Majestic, Supreme, Invincible, etc, are so fascinating.

If I remember right, Neil Gaiman's Sandman was partially based around refuting this.