Cross-genre books mix elements of two genre. The paranormal romance is really a romance with fantasy or horror worldbuilding. The sf romance is science fiction worldbuilding in a romance, etc., etc.
I'm a firm believer that you have to understand, read, and respect the genres you are mixing, or you shouldn't write it.
In recent paranormals romances I've read, the author didn't have a clue about fantasy or that you shouldn’t steal a prominent writer’s worldbuilding because it is blatantly obvious and annoying. One had a magic system that was a generic mishmash as well as a complete HIGHLANDER rip-off with swords, decapitations, and magic being transferred.
Another took the Harry Potter universe with its magic system and world, then tossed in her characters.
I've read futuristics that were really Klingons in love with the alien and STAR TREK names changed, or the science was so bad a third grader could have spotted the errors.
The danger of not understanding one of the genres is writers lose parts of their audience. Cross-genre is not only supposed to mix the two genre, they are supposed to mix the two audiences. Insult half that audience by not knowing your stuff, and there goes sales.
By ignoring the basics of the other genre, these writers are destroying “the dream" of the books, and that bothers me a great deal as a writer and a reader.