I have been using butterfly bars for the last few years. The good thing about them is that you have a lot of hand positions and you can use standard Mtb brake levers and shifters. The problem with butterfly bars is that on gravel or rough surfaces the place where you grip, while staying in reach of the brake levers, is too narrow to offer proper leverage. So, in my experience, butterfly bars are fine on roads and good surfaces, but not so good on rough tracks. This especially comes into play going downhill, where you want to keep your hands on the brake levers.

Magnus Thor
Iceland