It has some annoying misfeatures. Closely related types aren't treated as the same and you can't put multiple related types in a _Generic association list. You're forced to resort to explicit casts or call the type specific function directly which subverts the intended magic of using _Generic in the first place.

Some of this was fixed by DR 481 [http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr...], can you give some specific cases/examples at issue?

it's ugly and doesn't feel like a first class citizen of the language

This is a vague and emotional argument to a technical question.