Brushes are great with washboards! The only downside I can think of is that the metal ones can get stuck between the wood and the metal rubbing surface when you play which is a nuisance since that can damage the brush as well as screw up the song you’re playing. For that reason I’ve been using nylon brushes because they aren’t as fragile; they don’t sound quite as nice but close enough. I’ve also put tape along the bottom of certain washboards, covering that gap.
However once you’re playing with…