With scale outer an easy trick to eliminate the space that is visually wasted is to use larger images for your UI layer that exceeds the screen size. That way even if you have wasted space at least it looks like it is space that is being used. The other option is to add extra info (UI elements) in that area to display more info like high score, etc... I always use scale inner but my games so far have been more platformer style so it hasn't caused me any issues. I don't have things that should not be shown on those particular games.