I have been experimenting with artbushes composed of various forms and colours. I have noticed that particularly at small scale, the brush wanders,sometimes wildly from the correct path, please see the attached illustration ... I have outlined a "W" ... you can see that the brush behaves very differently on what would seem to be similar sections of the character. I have included the brush components between the Ws. The more complex the brush, the wilder the aberrations.
I use CS3 on Mac OS 10.5.1 but it is the same using CS2.
I don't need a fix, in fact I really enjoy some of the outcomes ... but really interested to know if anyone knows why this happens.
In case of converted type, you often get anchor points/ spline knots very tightly together. This will result in incorrect interpolation due to the spline tangents overshooting. That's reason one for pinching and some of it can be eliminated with a little cleanup. Reason number two is the simple fact, that art brushes are simply treated as wrapped deformations along the perimeter of the path. The algorithm used has no knowledge about the internal structure of the shape. For that, you'd have to use something similar to the object warps - associate the deforming vertex of the (imaginary) cage with the vertices to be deformed based on proximity/ range of influence of an imaginary forcefield.