Brake override system

The US National Highway Traffic Safety Administration is considering making brake-override systems mandatory for all cars. Do any of you know the potential drawbacks of such systems?

As you know the auto-cruise system in all cars already incorporated brake-override system: braking will automatically cancel the speed control.

My question is why the brake-override system is not implemented in most cars to avoid unintended acceleration. I believe such brake-override systems must have some drawbacks. Otherwise all auto makers would have put such systems on their cars many years ago.