Hip and knee replacements certainly out number shoulder replacement surgery. For example, a review of one East Coast state's hospital discharge records showed 994 shoulder replacements. This was compared to 15,414 hip and 34,471 knee replacements during the same time period.
So you're right that the hip and knee replacements far out pace the same operation for the shoulder. However, those hospital records were also compared for hospital length of stay, complication rate, and costs. They found the shoulder replacement surgeries had a lower complication rate with a shorter hospital stay. Costs were less, too.
It appears that shoulder replacement surgery is as safe as the more common hip and knee replacements.