Chinese characters can be used as a writing system because they're actually highly abstract. They don't look like what their ancestors were supposed to represent. That's a feature, not a bug.
The issue with emoji is that they're very literal.
The irony is that I think 囧 is the only emoji that is allowed on HN. Seriously, though, there are plenty of very old out of use characters that can be used as emoji, and modern use of Chinese characters in new words is increasingly phonetic.