"It's easy to be a good parent to a great kid. It is MUCH harder to be a good parent to a kid with difficulties."
You nailed it. You can also have a very disturbed kid who can act pretty darn normal on the outside. The kids who act out at least vent their feelings. It's the quiet ones that often blow without any warning.
Not that this implies the quiet ones are more likely to blow than the troublesome ones. Rather, since warning signs would often cause kids not to be described as 'quiet ones', the statement would seem to boil down to "Kids who don't give warning signs are more likely to blow without warning than those that do give warning signs, since the latter by definition can't blow without warning."