yup, they wouldn't be spoiled but they would be soggy. pizza keeps better because it's baked. the only thing i can think of is to take the burgers apart and stick the pieces into separate ziplock bags for the trip back.

it's so much better fresh because the buns are briefly toasted on the grill and then the sauce is put on which warms it. then you have to figure that greasy burgers aren't the best thing to store.