Personally when I buy a computer, I spec it for a specific purpose. I wouldn't know what the purpose of a wall computer is, so it would be hard for me to say if it works well. I hear they're good for routers, low-load webservers / important secret file storage, and home automation. As for a NAS/fileserver, I just build those out of whatever old computer components I have lying around (probably parts from computers that got upgraded).
Now that I think about it though, I could plugin a usb monitor and have a functioning... whatever.