The artwork cannot contain gradients, blends, other brush strokes, mesh objects, bitmap images, graphs, placed files, or masks.
For art and pattern brushes, the artwork cannot contain type. To achieve a brush-stroke effect with type, create an outline of the type and then create a brush with the outline.
For pattern brushes, create up to five pattern tiles (depending on the brush configuration), and add the tiles to the Swatches panel.