The line is really fuzzy honestly. We have some universal pictograms that are known and reasonably well understood around the world and the way they are used is pretty much a writing system. An icon of a man or women on a bathroom door? Well you may write it in one of a million different styles (fonts) but the general idea is used around the world as a common writing system. I'd say that belongs in unicode.
The real problem is that the alphabets of certain writing systems are unbounded. Emojis are completely unbounded. That's the only reason to have concern with it in unicode. Unicode is a limited set by definition and emojis are an unbounded set.
In my opinion the job of the Unicode Consortium would have been to encode what has significant and organic usage. Similarly to how Wikipedia only includes what has significant organic and externally validated coverage. If they'd stuck to that mission the line would have been a lot less fuzzy.
The problem with that is, of course, that "significant" is subjective.
The modern Western society is very occupied with the questions of racial and gender identity, and it is generally accepted in that society that this topic is "significant". And since it's that society that the Unicode Consortium is working within, this explains how you get six different colors of "man-pregnant" emoji in the world where there possibly haven't been six different-colored pregnant men.
Significant is only subjective in the heat of the moment and not much in retrospect. What I am arguing for is that the Unicode Consortium should only add characters with what Wikipedia would call notability.
I would like to stress that I am not arguing against the addition U+1FAC3 PREGNANT MAN or U+1FAC4 PREGNANT PERSON, there are good reasons to add these, but do we need mundane arbitrary everyday items line U+1FAA9 MIRROR BALL? I'd say no.
