I think that like many bits, it would be only as good as the hands of the rider. And she had better have darned quiet hands. Actually, it would be an interesting test for all of us. Could we give direction, ask for flexion, a stop, a canter depart without pulling the bit right out of the horse's mouth.
I have always loved a D-ring snaffle and I've ridden with one so long that my hand reactions are pretty subtle and effective. It's a security blanket sort of thing. So years ago, I went out on trail with a very green thorouhbred mare, one of her first outings away from the riding ring. The headstall broke and the bit dropped out of her mouth and I was left with a simple headstall and no reins for a moment. Being essentially lazy, I didn't want to walk home, so I attached the reins to the headstall, put the bit in my pocekt and rode her on home. The interesting thing to me was that she was very little different bitless than she was with my all-time favorite security blanket bit. It was a nice object lesson for me at the time. Nancy