>>1051278
The main difficulty is that a standard VGA/DVI-A/DVI-I output signal (likely the only analogue outputs available on any reasonably modern GPU) will be using a displaymode clocked at~31kHz, on the assumption that it'll be connected to a fixed-frequency 31kHz computer monitor CRT.
Plugging a cable carrying this signal directly into a device like a consumer TV CRT or a PVM, which are almost always fixed at a 15kHz horizontal scan rate, will result in the image appearing doubled onscreen, side by side as the CRT has received 2 lines worth of colour data in the time it took it to move the beam one line horizontally.
As mentioned there are methods to output 15kHz 240p from a GPU https://www.repairfaq.org/sam/ffmon.htm, although some GPUs may not be capable of running at a low enough clock speed to produce this signal.
This is generally quite inconvenient for a primary desktop machine, though.
> 240p is generally too small for any modern DE or WM to be used comfortably. In terms of screen real estate, think less Windows 98, more Commodore 64
> Applications, emulations, games, etc designed for VGA graphics may not even fit onscreen, and may cause issues when forcing to exclusive fullscreen mode. Many windowed applications won't allow themselved to be resized below 640x480
> Multi-monitor support for a 240p CRT alongside a 1080p+ main display is riddled with issues, so it's not easy to have the CRT off to the side while using the main display for other tasks