"Go wrong, Mr. Potter? I certainly hope not." Professor McGonagall's face was expressionless. "After Professor Blake was caught in a closet with no fewer than three fifth-year Slytherins last February, and a year before that, Professor Summers failed so completely as an educator that her students thought a boggart was a kind of furniture, [...]

Of course - I'm not saying that it can't have poor effects, merely that it's unreliable.

And I have to say, I always assumed it was violence that was inflicted upon Umbridge, not rape. However, upon rereading, it certainly seems like you're right.

"She got carried away," said Harry. "By a herd of centaurs."

...

Since she had returned to the castle she had not, as far as any of them knew, uttered a single word. Nobody really knew what was wrong with her, either. Her usually neat mousy hair was very untidy and there were still bits of twigs and leaves in it, but otherwise she seemed to be quite unscathed.

Edited the above to remove her as an example, because that's certainly nasty enough not to minimize the effects of the curse.

And as pedanterrific said, Summers is a MoR prof, not a canon one. (I'm pretty sure it's a Buffy joke).

I guess - the time Voldemort got the job in canon, it certainly set him back. Still, Dumbledore does not seem to be acting like he realizes who Quirrell is.