Electrical slowdown in hybrids does not provide redundancy for emergency braking. The battery has to absorb like 500kWt during 1s, this is well outside what a small battery can do.
Redundancy doesn't mean it will bring the car to a stop as quickly as hydraulic brakes, it is just another means to bring the car to a stop safely. From personal experience I could safely get my car to a stop in many situations without using the friction brakes at all.
