Most won't fail...I guess there is enough safety margin built in to handle it. But spacers definitely put more stress on the studs. In my earlier post, I used the analogy of the human arm. The shoulder is the "anchor point", and the further you put the weight from the anchor point, the harder it is to hold up the weight, because of leverage. With a wheel stud, the wheel is pressed tightly to the hub...close to the studs anchor point. When you add a spacer, the wheel (and the weight/stresses that it transfers to the stud) are moved away from the stud's anchor point. (Using the arm analogy, the weight is no longer near the shoulder, but now in the middle of the arm.)