I think the Switch gets this type of handheld play right by having detachable controllers. I never use my Switch with the controllers attached to the side. Instead I use a separate pro controller when playing handheld mode. Maybe this doesn't matter if you can still use a separate controller.
Beyond that, it's not clear to me what this will support. If I can play Fallout, any of them, I will seriously consider buying this.
The website is pretty clear: this runs SteamOS 3.0, which is Arch Linux with a KDE Plasma desktop. You can run anything you want on it, including switching out the OS if you want, and it uses standard PC peripherals.
In other words: your wireless XBox controller (or whatever) should work fine and you can play Fallout whatever as long as it is supported by Proton or you put Windows on the thing.
Beyond that, it's not clear to me what this will support. If I can play Fallout, any of them, I will seriously consider buying this.