The thing with informants is they can lie. They can pretend the loyalists are dissenters and the dissenters are loyalists. They can tell stories just because they personally don't like someone. They can demand protection money. It's vastly more fragile and unreliable than a system based on technology.

Sure the engineers can make the system lie. And they probably will to some extent. But it's a much smaller number of people you need to keep pleased and policed.