I used the Safariland Model 19 for several years to carry a 4" Ruger Security-Six and a 4" Colt Trooper Mark III. It concealed well, but the an emergency fast draw had to be well-practiced. As has been mentioned, the moving the dominant hand to the offside during the draw tended to slide the holster to the rear... a barrel-chested guy or big-boobed babe would not be able to use it very well.
I stopped using after a cold windy and snowy day, when while jogging from my car to the restaurant, that big Colt fell out of the holster, slipped down the front of my parka, and I watched it go tumbling end over end down the sidewalk in front of me.
I also had one for a 2-3/4" Ruger Speed-Six, and never had a problem. Part of the retention is the fit of the grip frame into the holster, and the Pachmayr grips on the Colt didn't allow it to seat properly. I had a Tyler T-grip on the Speed-Six and it allowed the gun to seat better. A few years before Safariland discontinued the holster (and Bianchi their Model 9R spring-steel version as well), they started printing a warning in the catalogue (no internet back then) that the holster was designed for guns with standard grips only. The double elastic on the Safariland tended to get loose after a while as well.