It's not portable. It looks really bad on a terminal emulator that does not support emojis (at least half of the ones out there esp on Linux). There are dozens of emoji fonts which all look different. You never know what your user is going to see. If your users are only using macOS and Terminal.app, then it may not be so bad, but if you are building a command line application, then I should be able to use it from a text-only console on an old system, VM, or embedded device. Don't assume all your users are going to be using it from a Macbook Pro or Ubuntu.

Personally, I don't. I quite like things to be text, because text can be understood. Icons can be... learned, I suppose, but then they tend to be inconsistent between apps and even change depending on themes and whatnot - so in general, it's mostly like playing a game of Memory where someone keeps shuffling the pieces.

But the main reason not to use emojis would be that you have no idea how they'll look to the user. I tried to paste the example output from yubikey-agent into my terminal, and all I got was a bunch of differently sized squares. Very informative...

You should try to configure your terminal to use UTF-8.

Symbols are universally used for quickly warning/informing in the real world and if done well, are very intuitive. Ignoring them in the digital world would be going against human UX (but no one ignores them, of course, even very old CLis already used them, but with the widespread use of UTF-8 and Emojis with that, it just became much easier and better).

My terminal has been configured to use UTF-8 for nearly twenty years. Apparently I do not have any font with these glyphs installed though, and I don't think it's reasonable to expect that people do.

Icons in GUIs are commonly used for interactive elements. Most CLI tools are not interactive, they just produce some output and the user expects that output to be easy to parse and compatible with as many terminals as possible.

You can easily output tables, bullet lists and many other things just with basic symbols supported everywhere. If your CLI program requires installing fontawesome or breaks in a terminal multiplexer etc. I'm probably not going to use it.

Emojis are part of unicode. No need for font awesome to support emojis.

Visit https://emojipedia.org and notice how all emojis have their own code point :)

I merely mentioned fontawesome as one of many possible examples. And as already said, a symbol having its place in unicode does not mean it is available on the computer or in a certain program. For example, in Linux terminals it's not uncommon that at least one optional font installation is required in order to get various emoji to display correctly, let alone other non-western symbols.

Many people use the terminal exactly because it displays fewer kinds of content than e.g. a web browser, which as a side effect simplifies many situations.

Being part of Unicode does not preclude having to install additional fonts.

Not all terminals support emojis. AFAIR, xterm doesn’t. I was stuck on xterm at my last job (only terminal that really worked on that system). Emojis are for SMS, and that should be it. Use emoticons.