I would agree that the HNL-ORD-HNL flight is tough to get upgraded. I've used RPUs and been 30th on the upgrade list on the day of departure.
I haven't really noticed a difference between LAX and SFO but maybe I ought to take notice. Seems like I clear CPUs about the same.
The hardest upgrade (albeit using RPUs) is EWR-HNL imho. Like everything else, YMMV.