I have been crazy lucky on upgrades for the last year...but as I often travel on lower fares (LUT), have had to use RUCs on heavy DM or FCM routes, e.g. ATL-LAX/LAS. I am typically still high on those, like 2 or 3, but perhaps with 1 (or 0) FC left. When I have higher fares, the problem is less. Occasionally I am 7 on the list with a LUT fare.

So yes, it makes a difference, but the flight characteristics itself seems to be much more important...how "heavy" is the demand.