I like them as an AI shibboleth, though -- the antennae go up, and I pay more attention to what I'm reading when I see it, so it raises the bar for the humans that ostensibly ought to be better at writing than the rest of us.
Edit: Interesting. I tried using -- and it doesn't work for me. I'd have to go change settings somewhere, or switch the browser I'm using to elicit an em-dash. I don't think I've ever actually written one, at least intentionally, and it wasn't until today that I was even aware of hyphen-hyphen.
Edit again: I had to go into system settings and assign a compose key — after that, I can now do em-dashes. Having degrees° will be nice, too, I guess.
The reason "--" autocorrects to an em dash in practically any word processing software (not talking about browsers) is that that's the accepted way to type it on a typewriter. And you don't need to go into any system settings to enable it. It came in around when things like Smart Quotes came in.