The file structure it requires. It's hard to pull off without breaking your torrents and after that it still will arbitrarily decide a file isn't the type for the library you're trying to import it into.
For a couple years I was getting around this by just keeping all my stuff in a single library and using the collections feature with all its options to make my own categories for standup, TV, and stolen YT videos. But today not even that lax of a setup could allow for some foreign films I wanted to add. It will just ignore movies or shows that it REALLY wants you to create an additional "other media" library for with no metadata