In the Cowboy Action Shooting world I have heard of the occaissional transfer bar breaking, but it is almost invariably in a gun that has been dry fired a HUGE amount. Most CAS practicing involves dry firing, and some guys do it to excess.
As far as quality is concerned, do not be fooled, investment casting, which is the process that Ruger uses, produces very high quality parts. Just look at their frames, Ruger uses investment castings for their frames, as well as many small parts like hammers, triggers, transfer bars, etc. Don't confuse investment casting with die casting, they are very different. With a milled part you would be sure to get stress risers in sharp corners, and that is where parts tend to break.
Just think for a second about what the transfer bar has to do. It is slammed by the hammer every time the trigger is pulled. The stress involved is very high. When the hammer strikes it, the transfer bar is not fully supported, it is only supported at an angle. It is really a wonder that they break as rarely as they do.
I used to know a guy who worked at the Ruger single action line in New Hampshire a few years ago. He took a Ruger and put it on a test stand that mechanically cocked the hammer and pulled the trigger. Did it thousands of times and the transfer bar was fine.