only works for a small range of settings & themes,
This seems sort of silly; just going by games mentioned in this thread alone we've got a fantasy game, a horror game, a physics platformer in a pastoral setting, a rhythm game about tribal warfare, and an action-puzzler with an Asian sort of theme.
I forgot about Silhouette Bros.
- it's a hack of SMB that turns each level dichromatic. Nor do games need to use only a few colors - Blade Warrior has beautifully-designed backgrounds that complement the gloomy, sharp-looking shadows of the foreground. Jumpman
could also be considered to be entirely silhouetted in various shades, without even backgrounds - but try looking at Jumpman and telling me it's not creatively designed and unique-looking! Each one of the games mentioned so far looks distinctive and stylish. (I think stylish is usually the word people use to describe silhouettes in indie gaming)
1-bit games, like Hero or its sequel, could be considered to be on the fringe of silhouettes. Not only do they have to contend with the drastically limited palette, but they have a minute resolution as well. I'd say they look fantastic, and surely someone could make a different 1-bit game and not have it look anything like Hero!
Now, of course silhouette styles are harder to make distinguished than full-color styles, but indie gaming in general tends to frequently set graphical limitations for itself. It's no different from limiting yourself to a certain small palette or tiny resolution. Or, speaking of homogeny, think how many games use Oryx's fantasy tileset!
Saying that silhouettes aren't "versatile in the slightest" is just a case of not using your imagination enough!