That's like saying that a scorpion in your boot is a threat because you were too dumb to check it before putting it on.
Whitecloaks are only a "threat" to sisters who are unprepared or unaware of them. I am not arguing against the book here. I am arguing that you are not taking the quote you keep posting in the correct context, and are over inflating the amount of "danger" the whitecloaks presented. The quote simply suggests that they can be a danger if they get the jump on a sister or attack them from ambush. It doesn't imply that the sisters actually consider them a serious threat.
They were considered a "threat" the way an unwary traveler might consider a pack of wild dogs a "threat", as opposed to something actually serious, like a band of armed bandits.