A cable replacement stays simple when the drum and track are in good shape and the spring's still doing its job. It stops being simple when the cable snapped under tension and let the door drop hard, because that kind of drop can bend the track, crack a bottom panel, or chip the drum itself. At that point you're not buying a cable, you're buying a cable plus whatever it took down with it.
It also gets more involved if the door's been upgraded since the house was built - a heavier insulated replacement door on original '90s-era torsion hardware is a mismatch we see occasionally, and it puts more strain on the cable than the system was sized for. If that's what's going on, a straight cable swap won't hold for long and we'll say so instead of selling a repair that's going to fail again in six months.