My experience with dirt is that any time I remove a plant, I get some floaters. However, nutrient-wise, dirt, in any form, seems to work well. And pretty much anything on top of dirt works pretty well.
But as far as total depth is concerned, 3" is generally agreed upon as a good substrate depth. In my experience, less than 2 1/2" or so is a problem for stem plants with big root systems - they're hard to bury, and the roots will occasionally pop up out of the substrate.