Among crashes where a "vehicle defect" is identified as the critical reason for a crash, brake problems account for 22% of them. It is a normal failure.
Both of these things can be true. Brakes can be the most resilient component in a car, but since they are so important, failure usually guarantees a crash.
It's nuanced. The brake system has several failure modes. Dual reservoir master cylinders would result in only half of the system to fail. You would absolutely be able to stop a car using only front or only rear brakes under normal conditions.