Mastectomy is very safe surgery, and most patients recover well with no complications. As with any surgery, however, there are risks. Possible complications are listed here, but keep in mind that unless stated otherwise, they usually do not happen.
The risks of any surgery are bleeding, infection, and injury to nearby tissues. Some post-operative pain and soreness is expected, but can be effectively treated with pain medication. There will also be a scar on the chest wall. Scarring occurs with all surgery, and is unavoidable.