My experience with brushes mirrors what I see as a common problem with any tickling implement, including fingers. If you are too hard or fast, or both, chances are good that it will create more pain than pleasure. Hurt rather than tickle. When I was an inexperienced tickler, or too eager to get to it, I would apply too much pressure and go too fast.
It also depends on the lee. Some find brushes, applied in the right way, devastating, while others are ho-hum. The same goes for electric toothbrushes, flossers, pens, pet gloves, etc.
My experience has been that lighter is better, getting both ler and lee to where they want to be. Unless, of course, you are into the more painful aspects of BDSM.