I believe it has to be a GTK app - which almost none of the things I use on a daily basis are. Try in gedit or something like that.

My daily drivers are IntelliJ, Firefox, Chrome, and Terminator so it's not super useful to me... :'(

Check out this extension: https://extensions.gnome.org/extension/1162/emoji-selector/

It implements a system level emoji keyboard that you can trigger with <Super + e>. It works pretty great and on all apps, I use it a lot.

Oh wow, you are totally right, that does work in GTK apps. My daily drivers are about the same :(