At its worst, using mixed media produces paintings at the level of "looks like the dog's breakfast". The result includes pointless texture that doesn't enhance the subject matter, clashing elements that irritate the viewer's eye, extraneous fragments from magazines that distract the viewer into wondering where/what they're from, garish colors that clash, almost inevitably pieces of mirror, and so on. As many things as possible together in one work. All of which looks precariously stuck on and in danger of falling off the support at any time. Not much better than that piece you made in kindergarten with the pasta, lentils, poster paint, and glitter glue.
That's a bit harsh, I hear you say? Well, is it? Who hasn't seen mixed-media work offered for sale that makes you wonder quite what the artist was thinking? (Never mind some of the stuff presented as 'artworks' in galleries!) That looks like the starting point was the artist's trashbin, and that the end point should have been too. It's this level of stuff that gives mixed media a bad reputation, one it doesn't deserve, because used well, the results are breathtaking (for the right reasons).
At its very best, mixing mediums uses the properties of the individual media so they work together, enhancing one another to create a result that's beyond what you can achieve when working with only one. If you think that no mixed media piece could ever be good, remember that it can be very low key. And includes things you possibly no longer think of as in this category. For instance, there's pen and wash, contrasting the starkness of a pen line with the delicacy of brushed-on ink or watercolor. The gold leaf used by Klimt in so many of his world-famous works. Using oil paint over an acrylic underpainting is also, theoretically, mixing your mediums! A watercolor painting where the initial pencil drawing is an integral part of the work, rather than a soft, mostly hidden starting point.
The photo I've used as an illustration on this page is a painting of mine. (I'm not about to put someone else's artwork on a page where it might be inferred that it was meant as an example of a bad mixed media piece!) It's done on paper, acrylic paint, pastel, and water-soluble Inktense blocks used both dry and wet. The intention with using the dry medium over the paint was to use the contrast in the mark making to echo the difference in texture between the tree and its surroundings. For it to stand slightly separately. To use the inherent texture of the paper (paint would simply fill in the small gaps, but using a pastel on its side means it catches only on the tops). It's mixed media at the beginnings of possibilities, not at the extreme end.
For adventurous mixed media, remember to pause to consider whether what you're going to add is going to enhance the piece, or whether it's going to detract. Don't clutter up a piece simply because you've got something you'd like to use; save it for another piece. Say it's an appealing bit of knitting wool that would create an interesting line, ask yourself whether the dimensionality (and the shadow this casts) adds something or distracts the eye? Will a line done in graphite pencil (which is shiny) or charcoal (which is matte black) work better (see graphite vs charcoal)? Or perhaps one scratched into the surface? Less glamorous, certainly, but it's about what's good for the piece, not about using up materials. And remember, sane people don't collect their tummy button fluff on the off chance that they can use it in a master piece.