Agreed. Audi Field, IMHO, is the difference-maker. I can't imagine the team would do well if in FedEx or RFK (without massive RFK renovations).
I heard that several of the other teams tried to play at soccer fields but were rebuffed by the soccer clubs who I guess didn't want to share the field or have it damaged.
I turned on the game from New York yesterday and the optics were bad. They're playing at the Meadowlands and the stadium is simply too big. The Defenders' crowd would have looked small at FedEx. For that matter, RFK was too big for DC United most of the time, so it would have been too big for the Defenders as well (setting aside RFK's decrepit condition and impending demolition). Seeing a smaller-time league playing in an NFL stadium reminded me of the terrible optics for the USFL when the Los Angeles Express would draw 10,000 people to the LA Coliseum back when the venue seated 95,000 people. There was no way to make a crowd that small look good in that stadium.
I can understand why some soccer teams' management might be concerned about field conditions. On the other hand, I gather DC United were very happy to have another tenant to help pay for the stadium, and on TV it looked like the field held up pretty well this past Saturday.