The biggest difference between a brush and finish mower is that brush cutter blades are mounted on a pivot so that they can swing away, should something solid be struck. Finish mower blades are fixed, hence are more likely to bend/break/hurt something when that unknown object is nailed.
For known areas like mowing a field that has been regularly cut, it shouldn't be an issue. But for true brush work, stick with the right tool.
Mike