Emoji hardly sounds like a good idea. I'd be curious to see whats the distribution of choice for different emojis. I willing to bet that most of the emojis will be from the common used set of emojis.

The problem with phrases is that people are lazy to type them. For this reason, it would be cool to see more projects that allow native authentication methods on the web, e.g. finger scan.

try out the service first, your facts will change.