The straight blade doesn't cut the sides of the wire insulation, and on wires with thicker insulation (600V class, as opposed to 300V) it tends to tear rather than cut, as you observed. The PTFE V-shaped blades cut the insulation on four sides, minimizing the tearing. The standard (black) cassette doesn't work at all on PTFE insulation.
In general, with this tool I found it needed some adjustment of the strip depth (the +/- slider at the top) before it would give a reliable cut for a given wire gauge. Once set, though, it works like a champ.