I used to ride bareback, especially when I got my horse from the furthest part of the pasture from the gate. I was very fortunate, my horse had a pretty comfortable sitting trot, he was not very bony when young, and at that time my MS had not destroyed much of my central nervous system. I always put my helmet on before I rode bareback.
M. Horace Hayes said in one of his books (Stable Management and Exercise"?) that too frequent bareback rides could lead to callouses on the horse’s back from the rider’s seat bones, and that these callouses could negatively affect the fit of the saddle, even if the saddle had fit the horse previously.
So the summer when I just rode bareback I got up into a Forward Seat as much as possible when riding him (one to two hours a day). He did not get callouses on his back from those 3 months of riding bareback.
That summer was really hot and muggy down here in NC. I found that I had the most security bareback if I rode in a generous wrap around skirt (thin cotton from India), hardly “proper” riding wear. My bare skin against his hair coat gave me excellent grip, otherwise I tended to slip around his back.
Now my balance is so horrible from my MS that I refuse to ride bareback and I also refuse to ride without stirrups. If I felt secure enough to ride all 3 gaits without stirrups I would go back to riding bareback periodically, but since the last two times I fell off the horse was at a walk I decided not to be stupid about it.