I use paracord quite a bit, with several knots styles. In general, I do not have a problem with knots slipping, but I have come across one "knot" problem.
That is with a slip/tension style knot. If you go to hang a ridgeline or clothesline or whatever, and for tension you use this style of knot, and then load the line (clothesline) I find that the knot holds well, but moves the outer sheath with respect to the inner strands and will bunch up and allow the overall rope to sag. If that makes any sense.
My solution is to either use a truckers hitch or a power cinch type knot for the tension in that particular case.
I still use paracord as the "kit" cord, but there are plenty others I like for general use as well.