I think my fear of breaking off a heel, especially for a stiletto, has been the greatest concern that "prevents" me from wearing them. I understand that it is the quality, and thus, strength of the shank and not necessarily how well the actual heel is attached to the shoe that may result in breakage. One of the "issues" I have noticed about stilettos is that of what I call "heel flex." I have and had stiletto heels in which, when the bottom of heel makes contact with the ground, it firmly sticks in place but as the foot bed comes in contact with the ground, there is a slight bend of the sole, pushing the top of the heel slightly backwards while the bottom does not move. When the foot is rolled on to the ball of the foot, the heel "flexes" backwards. My concern that the repeated flexing/bending, ever so slight, will eventually lead to a fatigue failure and the heel becoming detached from the sole. This flexing/bending, however, is not limited to stilettos as I have noticed it happening to other heels, even block heels. I am not sure if this is "normal" or is a reflection of the quality of construction of the shoe/boot.
I have learned that heel breakage is not limited to stilettos or other taller height heels. Some time ago, I was shopping at a larger retail store while wearing a pair of "flat - heeled" riding boots. Shortly after I entered the store, I noticed that the 1" heel on the right boot was making a strange sound. As I looked at the right heel, I noticed that it was barely attached to the sole as the glue holding it in place was minimally applied. I removed it, placed it in my pocket, and hobbled around the store quite embarrassed by the situation. When I returned to my car, the boots came off and I tossed them into the back seat. When I actually returned to my residence, I noticed that the left heel was starting to come loose from its sole and what followed was a sting of profanity that, last I checked, is still hanging above Springfield, Illinois, and has been known to cause some degree of turbulence for air traffic in the area. What made this situation infuriating is that the boots were a designer brand from which this type of nonsense should not be expected. I glued both heels back to the sole and sank three small screws into each heel to help secure them in place; I had no issues with the heels when I last wore them. The point to this is, what has been echoed in previous replies, materials used and craftsmanship of the product may be the ultimate factor when/if a heel should become detached from its sole, is right on point.