Sure I know that, but in Slack and lots of other applications I can just type \:facepalm\: (without the \ of course) and I will get the 🤦 emoji. Works in this forum as well btw.
In messages, it doesn't. Plus, when I use my Mac in German, I cannot type the common English names of the emojis in the emoji picker. I have to type "nichts böses sehen" instead of "see_no_evil". That sucks. Again that works perfectly on Slack, WhatsApp and others, even if my Mac is running in German language...
Having that said, it would be nice, if Messages supported Markdown...