In the states where I practice, there is no immunity afforded to the complainant in a board action. So in the end, the statements given to the medical board must be true. Absolutely true. Truth is a complete defense to defamation.
Most states have protections for those reporting medical errors. Truth is also a defense to any defamation action. If a doctor sued he/she would have to pay a lawyer and the very issue he is worried about discussing would be litigated and would likely shine more light on the issue than ever. No reasonable doctor would get involved in such a case.