Universes Beyond is a part of Magic the Gathering where Wizards of the Coast collaborates with other popular brands like Lord of the Rings, Marvel, Star Trek or Final Fantasy to bring those “out of universe” characters and worlds into Magic and mixes them up with “in universe” or Universes Within worlds.
The “superhero problem” stems from this idea that players want to buy these sets to play with their favourite heroes and villains (I’m using the term “superhero” flexibly here: an Aragorn, Captain Kirk and Doctor Who are all “superheroes” under this idea) and all of them need to be powerful and exciting. More pressure to do it comes from the owners of those intellectual properties (or at least I assume so based on how capitalism and self-exceptionalism works in this world) who want their characters to be portrayed in the best possible light so it doesn’t undercut the original brand.
In his video essay Why New Magic is Hard to Watch, William calls it “No more nobodies” and gives a great example of the problem:
A random background soldier in an in-universe set can just be a 2/2 with Vigilance because nobody expects more from it. But if you’re turning a famous superhero into a Magic card — someone with signature powers and iconic moments — all of that has to be represented on the card.
The designers are well aware of this problem. In Sneak Into Design with Magic: The Gathering® | Teenage Mutant Ninja Turtles, designer Eric Engelhard writes (emphasis mine):
Universes Beyond sets want a player who is even slightly familiar with the property to recognize something in every booster. That’s sort of the promise of Universes Beyond: we’re taking a universe and expressing it through Magic: The Gathering. If you’re a fan and you open a booster of a Universes Beyond set and you don’t recognize any creature, that’s kind of a failing for Universes Beyond. We’ve found both new and existing players feel that way. So, we want recognizable creatures at low rarities, but those are the very ones we can’t make nonlegendary, because everyone knows them. We couldn’t put a picture of Leonardo on a creature card and call it “Youthful Altered Stealth-Inclined Tortoise.”