According to Tris, what makes Jeanine so terrifying?

Here is a quote from Tris:

I see no vicious glee in her eyes, and not a hint of the sadism I expect. She is more machine than maniac. She sees problems and forms solutions based on the data she collects. Abnegation stood in the way of her desire for power, so she found a way to eliminate it. She didn't have an army, so she found one in Dauntless. She knew that she would need to control large groups of people in order to stay secure, so she developed a way to do it with serums and transmitters. Ch 34