Yep, goose is quite a dark meat, and is easily overcooked to dryness. But potatoes (or anything) roasted in goose-fat should be the highlight of anyone's christmas dinner!
If swan is anything like goose (which I can only imagine it is), it's probably too fussy for the average home meal anyways. Goose can be delicious, but it's not something you'd cook outside of an occasion like Christmas.