I want to add, that most digital monitors are vulnerable to Tempest too. It is a bit different in requiring software to attack instead of old television equipment, but the EM radiation is even higher from what i observed. Range differs with equipment, antenna, frequency, model of monitor and even displayed content. There is no way to tell how big the problem is without measuring it.
Any thing (peripheral, box, cable) that handles output from your machine should be viewed as mission critical hardware and if physically tampered by your adversary is able to extract data.
If your things are hooked up to network this opens the possibility for network fuckery (for example smart TV) and increases attack surface.
IMO having input ports like USB/SD card readers or something like that makes attacking devices arguably quick, but if your adversary has physical access, chances are he has plenty of other means to attack that can be deployed in seconds of unobserved access, like deploying cameras or microphones. Deployment of a mitm device in the output path may even be quicker than waiting for the monitor to boot and attack via USB, while giving the adversary the ability to use any hardware capabilities for example wifi that may not be present natively in the otherwise attacked device. Also such capture devices are compatible with standard ports like HDMI/VGA, while a USB exploit on monitors is very depended on actual device model and firmware version. More stealth, but much more costly.
