I've tried this feature out and found that it doesn't do as good a job of blurring faces as I'd like, especially when those faces take up more of the frame. I posted some pictures here:
Basically, I think they're using a constant blur size which fails to adequately obscure faces that take up a lot of the image because when a face takes up a lot of the image then the features of that face become large, which would require even MORE blurring to obscure. And they're not doing "more blurring" when the area which needs blurring grows, or at least they aren't doing enough additional blurring.
http://lelandbatey.com/projects/signal_blur_comparison/
Basically, I think they're using a constant blur size which fails to adequately obscure faces that take up a lot of the image because when a face takes up a lot of the image then the features of that face become large, which would require even MORE blurring to obscure. And they're not doing "more blurring" when the area which needs blurring grows, or at least they aren't doing enough additional blurring.