One item that seems to have been mentioned once but then glossed over is that on an upright, the strings are both flatwound, with extremely small space between wraps, and therefore very smooth. That makes it unlikely to to do damage to a nut that may be brittle. A roundwound is a lot like a saw (and can be used as one in some applications), and the event of one relatively coarse winding wrap being pulled over the sharp edge of a nut, under pressure, would have a far greater likelihood of doing damage to a brittle material than would a smooth flatwound.