In some circumstances, it can be hard to change people's minds. In others, it can be way too easy. For example, look at bullying in schools: a large number of people somehow come to agree on bullying a certain set of targets, even when they probably would never be so cruel on their own. Or look at political propaganda, when given by a trusted source and decked out in applause-lights-inducing standard rhetoric -- preaching a slightly new message to the choir. In both cases, the people being persuaded just weren't adequately defending themselves from the influence of the people around them.

An obvious example of a case where it's hard to hack somebody's mind is trying to get someone to stop being racist. That's hard; it's easier to get them to stop being racist in public, by making them ashamed of it.

So, sure, the Dark Arts are not all-powerful, but they're not weak either. And they are always creepy.