I use them regularly, the brushes I use have
fine soft bristles, and are pretty gentle but
still there is definately a learning curve. Some patinas will tolerate moderate scrubbing with little to no visual impact, others will be stripped with the lightest touch. Light blue green
patina for example will brush right off with a
brass brush, where some nice deep green ones can be scrubbed pretty vigorously without harm. Unfortunately you will probably ruin a few coins in the process of learning what can and what can not take the scrubbing.