As a college student, I use emoji constantly to communicate all sorts of abstract sentiments, but in my experience they can also be irritatingly ambiguous and highly dependent on cultural norms and interpretation.

Take the thumbs up emoji - within my social circles, the exact same emoji can be interpreted both as a enthusiastic agreement ("Sure!") and also as a sarcastic affirmation ("Good for you.").

It's often difficult to infer the intended meaning, even with context, and in some circumstances I've found emojis have actually added significantly to the ambiguity and cognitive burden in parsing a text. That's not a problem I have often faced with simple smileys.