I can't find the information, but my understanding is that this approach isn't good from an accessibility standpoint because a screenreader still interprets the icons as their corresponding character and reads that character. As a result, instead of ignoring a checkmark icon, it would try to pronounce it as a single letter.

UPDATE: Further digging seems to validate that most modern screen readers do ignore pseudo elements. So, there's no doubt that this solution is promising. However, as with anything, your user agent baseline should be considered before using it. Saying that everyone should be using this right now probably isn't a reliable blanket recommendation.