Uber is easy at IAD, and because of the taxi monopoly they almost always have taxis too, although in experience that's more expensive. (We once arrived on delayed flight at 3am and there were taxis).
As for using Uber at IAD, you'll be asked to select a door when you exit from the baggage claim area (it's slightly up, where cars pick up, not down) for the Uber driver to meet you at (the app will ask for a choice). FWIW, Door 1 has always been a madhouse for me, I assume because that's where international arrivals come out.