After NW merged with CO, as a NW Plat, my success ratio on operational upgrades (free due to oversold Y) took a nosedive. This year, however, flying on K or V, my wife and I have been upgraded to C more often (50% success rate) than ever. This has more to do with NW's current high pax load than anything else.
Recent Asian travel to HKG yielded nothing, PVG was 100% in C, SIN was 50%.

By definition, an operational upgrade occurs ONLY when coach is oversold. Otherwise you must use miles to upgrade on applicable fares.

BTW, our domestic success rate remains 100%.