That was my concern, too. There's a precedent for a radiation-emitting machine failing in a very bad way: https://en.wikipedia.org/wiki/Therac-25

Of course these new machines cannot fail the exact same way as the Therac 25. But could they fail in a new, unpredicted way? I dunno. I am not familiar enough with how the backscatter machines are implemented to be able to predict how they could fail. But I maintain neither are most of the people who defend these machines as safe.