I think in Simon Cowell's world harmonies mean 'oooh' repeatedly so I feel bad for Niall and Louis. Even still, I think they were very good, and definitely not the wost of the night (IMO, that was shockingly Rebecca).
People on DS will never like them, so they make up miming as a justifiable reason. It's like the show is not fixed, just manipulated. Do you really think Simon Cowell would be stupid enough to make them mime? Poor fools. In the same way as they criticise girls for only liking them because they are 'young and cute', I get the feeling that they only dislike them because they are 'young and cute'.