When it comes to brakes, the first three priorities are reliability, reliability, and reliability. If indeed the brake-by-wire is as reliable as it's cousin, fine and dandy. But I've worked on brake systems and I, to be blunt, like the idea that if you slam on the brake pedal, oil travels through lines and closes calipers upon brake discs. You can do this with a car turned off, with it's engine disabled, with it's electrics completely dead, because the linkage between your brake pedal and the brakes is a mechanical one.

I'm not saying there aren't ways to make this equally safe with software, I can think of many ways. Do I necessarily trust the built-to-cost auto industry to do them? Ehhhhh....