Generally speaking, a Mary-Sue character is harder to do well than most other types of characters, because it has to be done extremely
well to be worthwhile, but if it is
done extremely well, the results can be quite good. Just as it's very difficult to write from the point of view of a radically different species, and end up with anything worthwhile, but if you can pull it off (see Alan Dean Foster's Humanx Commonwealth prequel, Nor Crystal Tears
), the results can be downright spectacular.