There are ~1800 widely supported emojis. That blows out the search space. There are 10^19 possible passwords for a 6 character alphanum+emoji, the same as a 10 character password using just alphanum + simple keyboard symbols.
But what's the actual distribution of usage of emoji? I'm guessing there is a small-ish set that people actually use, just like there is a small-ish set of words that people will pick from if you ask them to pick a passphrase containing "random" words.
Unless you pick the passphrase for them, you're not really gaining the advantage of the search space size.
Except people will probably stick to a few common ones and do a pattern. Love eyes x2 heart heart, thumbs up -- just like people don't pick words uniformly.
Unless you pick the passphrase for them, you're not really gaining the advantage of the search space size.