[Jonnax]

It could be argued that Chinese has a lot more characters to learn if you're including the Nanori (Readings used in Names) for 生.

If you take a language like English, it's still arguable that you'd need 12 years worth of schooling to do an advanced test.

Kanji Kentei seems to ask esoteric things at level 2. So it's a test of fluency of not only the Kanji but the language.

How would an English learner fare with Shakespeare or reading a book like Finnegans Wake?

I don't know if there's any data about this. But I think what would be a good study is the level of native proficiency in languages around the world.

Does languages considered simpler result in a greater mastery by the general population?

[Clewza313]

The Kanji Kentei is not a language test, but specifically a test about kanji, and the fact that you need to be familiar with Chinese poetry etc to ace it showcases how complex the writing system is. About the closest English gets is asking obscure loanwords in spelling bees.

[anthk]

So like reading the original unadapted Don Quixote in Spanish. Not bad as Shakespeare, but some words had a different meaning back in the day, with inusual metaphors impossible to know unless you were an expert on Middle Ages.

For example, "La negra" would mean "the sword", and "duelos y quebrantos" wouldn't mean "mournings and breakdowns", but some kind of dish made by mixing scrambled eggs, chorizo and bacon.