What are the downsides of getting a boob job?

Foreign body. The body reacts to implants by makin a scar around them, this scar can harden over time, and needs to be removed at some stage. Implants are not permanent, and need to be changed. You can have chronic pain, back pain, and sometimes even a poor cosmetic result, depending on your surgeon and also how your body reacts to the implant. Your nipple sensation can be reduced occasionally, and a scar.
Repeat surgery. Implants are not lifetime devices and you may need to have another surgery down the road either to replace them, go bigger, smaller or just remove them. There are also potential complications such as infection, capsular contracture and droopy breasts. If you have very large implants, your own breast tissue may atrophy under pressure.
Cancer detection. If you mean augmentation (implants), aside from post-operative discomfort, and sometimes need for procedures down the line; the only real negative effect is sometimes making examination and mammography more difficult; making cancer detection somewhat more complicated.