I used to use one, but ended up going back to a bit. She had problems with bits- HATED them. So I decided to work on that, and it paid off, she is great with one again. And responds better. I went to a snaffle.
I've always used a bosal for starting new horses. While I feel I have soft hands, my seat is not nearly what it used to be and I don't want to jar the mouth. Today I still feel more comfortable in a bosal. I had an accident with a mare where she wigged out and while trying to get her head around the bit was pulled through her mouth. Totally freaked me out, and I haven't been comfortable with them since. I still have an ultra correction bit hack combo that had an adjustable chain on the chin strap. If a horse through a fit or was disrespectful that bit would go on, a few quick turns and the attitude disappear. I no longer use it but I have a cowboy friend who has wonderful results with it. I've lost quite a bit of my fearless cow girl side and don't care to tangle with the green ones any more. I taught my daughter to ride with a bosal till she gets a better seat and she's doing great.