Like any intrusive government program, it is great until it is abused. And law of the Universe #3 states that: Any power that a government can abuse will, sooner or later (Probably sooner) be abused.
That’s the issue. It will be abused. And probably badly abused.
That said, in the PRC this stuff is already here. Almost all payments are made via electronic transfer. Doors to your house and car are unlocked by your cell phone. They just need to implant the thing in your hand and on your forehead.
Such a thing is designed to be abused.