There's the Swiss cheese theory of accidents. Each layer may have holes in it, but by having enough layers and aggressively trying to shrink the number and size

There's the Swiss cheese theory of accidents. Each layer may have holes in it, but by having enough layers and aggressively trying to shrink the number and size of holes in each layer, you can get much more secure. External audits by good teams are a good layer, but neither they, nor any other individual layer is a silver bullet. Good audits do provide a lot of value.