That's a good question. I haven't made many quilts, but the scrappies that I have (doll and baby size, mainly) made me ask the same thing. What can successfully mix? Personally I think there is a lot to be said for using a decent amount of solids (and by that I mean to include tone-on-tones) so the eye can rest. It's also about balancing darks and light values. I don't think it works well unless you have some sashing between busy scrappy blocks either. And I tend to gravitate to the aethetics of a quilt that uses the same "family" of prints, but I can't say that's always the case. Pattern has a lot to do with what works. I do squint my eyes when I'm not sure if something if working. And if I don't like a certain scrap, I won't use it just because the color, scale, or value is right. I want to smile at every piece I put into a quilt, not feel apologetic for a single one of them.