I actually have had worse luck with u/g's coming ex-SFO and ex-LAX on my way to IAD than the other way around (failed to clear twice vs. never failed this year so far).

As others have pointed out, the equipment type and the factor of these being nonstops are big reasons you're not clearing often. I find that connecting almost always solves this problem, but then again, you do add the inconvenience of a connection, and at times, a much-reduced meal service for the u/g depending on where you connect.

As for getting UGS, that shouldn't make much difference on 500's... what's the UGS window, 120 hours? So you'd have a chance to clear not quite a day ahead of 1Ks? Probably won't mean much, unfortunately.