The abortion pill has an over 95% success rate; medical abortion is generally considered very effective.
However the abortion pill is not without its share of possibility of complications and in the event it does not work, the pregnancy would have to be terminated by a surgical or aspiration abortion.
If the abortion pill does not work and the pregnancy is not terminated then there are chances that this could have a very adverse effect on the surviving fetus.
The likelihood of the pregnancy culminating in a healthy child are very slim; the fetus is likely to be severely deformed if the abortion was partial or incomplete; that is to say that part of the pregnancy was still left in the uterus.
The woman could also be put at risk of certain complications that could arise from the abortion pill: there could be an allergic reaction to the pill; there are chances (however slim) of infection. There could be heavy bleeding following a pregnancy terminated by the pill. And there could still be the chance that there is an undetected ectopic pregnancy.
In very rare cases serious complications could result in a fatality but the risk of an abortion pill causing death are far less than fatality resulting from childbirth.