Radley Balko explains why they're biased:
In too many jurisdictions, medical examiners report to the attorney general or to the state official who oversees law enforcement. In states like Mississippi, where for most of the last 25 years prosecutors contracted criminal autopsies out to private doctors, the incentive for medical examiners was to produce results beneficial to the prosecutor's case. If they brought back results the prosecutor did not like, they risked losing future referrals. As I've reported during the last several years, that system produced the travesty of justice that was Steven Hayne, a physician who testified in thousands of cases despite serious questions about his qualifications, credibility, and practices. But if it hadn't been Hayne, it would have been someone else.
Although states where medical examiners work directly for the state are better, incentive problems still exist. There is always pressure, blatant or implied, to deliver results the state needs to win a prosecution. That does not mean all or most or even a significant percentage of medical examiners are corrupt. But having a medical examiner and his staff ultimately report to the head of a law enforcement agency introduces subtle pressures that can influence even the most conscientious doctors.