Whatever genre-mash is concocted, I've a Super Big Rule as a writer and Top Demand as a reader:
The novel better deliver on the marketed genre's expectations.
Your friendly Word Whores have written about those expectations before, so I won't rehash them. Sure, retailers ultimately decide where to display/list the book. However, as an author, I better be able to succinctly answer the most common question put to writers, "What kind of stories do you write?" "Plot Stew" is not an acceptable answer. "I write a little bit of this and a little bit of that," isn't going to help sell your book. You need to be able to say, "I write High Fantasy Mysteries" or "I've published a series of Amish Space Westerns."(hey, the latter could happen, right?)
If you're going to mix genres, do it well and do it for a reason. Tossing assorted genre tropes into your story may give you a lot of rope for creativity, but you're tying your hands to the expectations of readers of those genres.
Be mindful of the marionette.
It doesn't entertain when it's tangled.
