@rysiek Ü is quite a BIG mouth, small eyes, while :) is not so big, just wide. With :) proportions are better. Not to mention that Ü may look *very* different depending on the font, not quite like a smile, while :) remains recognisable.
Case in point: any Serif font will add cheeks to Ü, handwriting makes it non-smiley-like, some fonts will make the left and/or right sides bolder, and so on.
Not to mention that there are other variants, like >;), ;), O:), :(, and so on. It doesn't make sense to use Ü for one, and a rotated one for the others. Consistency matters!
@rysiek german here.
In fact I never thought about using them, probably for the same reason you should not write FOSS code with primary-german (basically primary non-english) commentary if possible. (I'd be willing to learn esperanto, butusing english as a bridge-language seems easier).
When someone uses the wrong font it also looks shitty and they can't know which smiley was used which leadsto potential confusing situations. And a personal reason is that I think :) looks "cleaner" :D