They used to sell a model that had an interchangeable deck. One for yards. One for brush. Don't know if they still do that.
If they don't, one issue is how low you will be able to cut with a deck that is typically set to mow higher when cutting brush. Another issue is the brush deck is designed to discharge brush/tall weeds. A brush deck isn't really designed to pull yard grass straight up, cut it, and discharge it in a neat pile.
IMO, the general issue with brush mowers is they are pretty expensive. You could buy a Cub Cadet ZTR1 with a 50" deck and Kawasaki engine for about what they are asking for a 30-32" walk behind brush cutter.
In my personal experience, walk behind brush cutters are a heck of a lot of work. If you have tall briars, you're going to get snagged trying to cut them down. I think this is why I see brush mowers listed for $400-$1,500 depending on their age because people don't like the work involved using them to cut brush.