So how many metadata standards is too many metadata standards? How many metadata frameworks is too many metadata frameworks? How many roads must a man walk down, before you call him a man? (Seven!)
EPUB has its own share of metadata headaches. On the one hand, you need enough centralized metadata to facilitate reading system presentation of your book to readers. On the other, you need metadata to move the publication through a distribution chain. And even then there are people who want MARC, MODS, JATS and other metadata to facilitate their own processing.
The EPUB specification sort of floats between problems neatly enough, but it’s yet another custom framework that in my experience causes as much confusion as it solves, and leaves me wondering if there were another way to handle metadata, like going all-in on schema.org in a future version.