Advantages:
It can make a girl more at ease with her body and increase her self-esteem.
The physical complaints that are caused by breasts that are (too) big normally disappear after a breast reduction.
Playing sports is more comfortable.

Disadvantages:
There are risks involved, as is the case with any operation. There may be an adverse reaction to the anaesthesia, the wounds can get infected and there may be inadequate healing.
The nipples can become numb and it can be more difficult to breastfeed when you get a child.
There is scar tissue. Around the nipple and in the crease under the breasts this is not very visible, but the vertical line from the nipple to the under part of the breast might remain noticeable.