Erlang/Elixir is great for internal command-line tools where a several-second startup time is acceptable. If you're looking to distribute the CLI tool externally, it's worth considering other languages.

BEAM would make a fine engine for GUI stuff but the toolkit bindings just don't exist. As another poster mentioned, LiveView + Electron could make a very compelling pair.

Erlang/BEAM has been used multiple times as a gaming network server language, but to my knowledge it has never been involved in the UI.