I didn ́t have enough miles to fly with another carrier and BA was the cheapest option to fly to Scotland
I have taken the United flight between DC and Zurich countless times in the past two years. I have global entry, so customs isn't an issue for me, but the lines never seem that long at Dulles. I mean, there are so many international flights there is usually enough people working. Its the actual turning in the custom form with baggage that usually takes forever, with usually only 2-3 people working.
The best part about IAD-ZUR flight either way is that its RARELY full. I've only had one person sit next to me in economy in ~16 flights. Usually I move and get a whole row to myself and sleep the entire time. And I actually really like United to be honest. Food is good, always treated nicely and flight time isn't that long. I have flown Swiss to Boston and I didn't like the seats as much (economy) compared to United, but the service was good, just like United.
So I recommend the flight between Zurich and DC with United! And usually its much cheaper than flying between Frankfurt and DC with United or Lufthansa.
Sunday & Friday flights are pretty busy whether you fly from Zurich or Geneva