Why are infinitely growing arrays bad?

Question

MongoDB

Why is infinitely growing arrays in mongodb a bad practice?
If we consider an example from the documentation , then why is it considered unacceptable to have an array with book identifiers in an Autor document and is it suggested that Book documents refer to the author? Is it really more efficient to search for a hundred books belonging to one author out of a billion than to select a hundred books by identifiers stored in an array? Logically, it is much more likely that I will choose all one hundred books before I list the whole billion. Or does mongodb somehow optimize this process?

Answer 1

and where does it say that Monga will be
?
an example in general about reducing redundancy
, but the answer is there at the link