I've had the Castwell knot slip with a furled leader once before, but only after trying to dislodge a fly from tree limbs. If you pull on it the knot will hold, but I wouldn't count on it to stay 100% of the time using some of the common tricks for dislodging flies from trees that involved "springing: the line or whatever you would call it.

I think monofilament "bites" into the flyline better making this much less of a concern when not using a furled leader.