Is the harm that the average ethical review board prevents less than the harm that they cause by preventing research from happening? Are principles such as requiring informed consent from all research participants justifiable from an utilitarian perspective?
Most of these sorts of meta interventions have not been tested on their own terms. (One can do a Bayesian update on Bayesian updating, and A/B test A/B testing, but it's much more difficult to use the Precautionary Principle on the Precautionary Principle, for example.)
So we should expect ethical review boards to be PR positives, because they were constructed for reasons of PR; I don't see any particular reason to expect them to be net positives when it comes to ethics.
Seems quite simple to me. We should never use the Precautionary Principle, because we cannot rule out the possibility that it would do harm. ;)