The problem: slice too hard, and you risk slicing off the casing of the inner copper wires. While not exactly the end of the world, it's better to avoid this.
Instead, run your cable slicer very lightly several times down the cable. The goal is not to slice in one stroke, but almost to create a perforation that you allows you to rip the casing off by hand.
For me, it usually takes three strokes.
