Retropie has a lot of emulators available to download.

A frontend to install and configure most of capabilities.

If you have the knowledge, it's possible to add unsupported controllers, unsupported software and more.

It runs on top of a full Linux distribution so you can tweak, replace, customize anything without much hassle.

You can use a minimal Debian testing and install Retropie on top of it.

Since you have a full distro behind you it is possible to run/integrate any application.

Easy to use.