The Super NES controller port is functional, and the device also successfully syncs with the Switch Pro Controller, which is how I've primarily been testing it. I've tried three games so far: Doom has artifacts, but Doom II works, while Super Mario Bros. 3 is also perfect.
I'm not sure what I'll ever use this device for, especially since my CRT TV overscans contemporary stuff like this, and the RetroArch output from my PC is so perfect and configurable: but it's still nice to have a modern way to play actual GBA carts on a CRT or HDTV, even if I don't have a concrete use case for that right now.