Horrible for: UTF8, syntax checks, numbers (albeit JSON has this problem too for floats), deep nesting, arrays, strings as keys, consistency between different parsers, etc etc
Good for: comments, readability if keys limited, familiarity.
UTF-8 is only a problem if you're using the Windows implementation. I don't see what the problem with numbers is, either, pick a representation that suits you and use it.
Good for: comments, readability if keys limited, familiarity.