I'm 19 and considering an endometrial ablation. What are the possible benefits and risks?

Too young. Endometrial ablation (destroying the uterine lining to reduce menstrual bleeding) is great if you are 45 and done having children but isn't for people under 35 (generally). Ablation should never be done if further childbearing (pregnancy) is planned because serious complications for mom and baby can result. Ablation does not provide pregnancy prevention, it isn't sterilization.
Other alternatives. While an endometrial ablation is an option for woman who have heavy periods, other factors such as the patient's age, whether she has completed childbearing and need for birth control need to be considered. If you have an endometrial ablation, you still need to use a reliable form of birth control as pregnancy is generally not recommended after the ablation. Talk to your doctor about options.
Infertility. You are not supposed to get pregnant after an endometrial ablation. It destroys the lining of your uterus. A pregnancy that occurs after an ablation often does quite poorly, and is not recommended. We generally do not recommend it on someone your age. There are other options for heavy bleeding, such as ocps or other meds.
Ablation. A woman should only consider an ablation if she is done having kids. A pregnancy after an ablation, even a miscarriage, can cause a life-threatening condition called accreta where a woman can hemorrhage severely. If a pregnancy continues after an ablation the baby can have severe growth problems or stillbirth. Ablations are really intended for women who are in their 40s whose tubes are tied.