What are the disadvantages of electronic shutters?
The most important potential problem with electronic shutters is the "rolling shutter" effect. Because the imaging information is read from the sensor pixels slice-by-slice, a very fast-moving subject can move during the time it takes to read the whole sensor. This results in the subject being distorted in the final image. A speeding train, for example, might be just halfway across the frame when the top line of pixels is read but near the edge of the frame by the time the bottom row is read. Consequently, the train will appear distorted in the image. The shutter speed (or, more accurately, exposure time) is still 1/8000 sec or whatever was set; it's just that each slice of the image is a very slightly different 1/8000th of a second.
Developments in sensor technology, such as the stacked design of the back-illuminated CMOS chip in the EOS R3, enable much faster readout speeds than before, greatly reducing rolling shutter distortion. Another new technology is the global shutter (or total shutter), which reads information from the whole of the sensor at once rather than line by line, but this technology is very complex, adds both image noise and cost, and can't yet produce very high-quality outputs, so although it is used in some video applications it is not practical for video or stills where image quality is the key requirement.
The flickering of some light sources, such as fluorescent and LED lights, can also cause banding when an electronic shutter is used because the brightness and colour of the scene changes during the period when the sensor is being read out. In a similar way, it can be difficult to sync a flash with an electronic shutter because most flashes produce a very bright but very brief light, meaning that the intensity of illumination is not sustained for the duration of the sensor readout. These issues, however, can also arise in certain circumstances with mechanical shutters, and the latest technologies in the EOS R3 include high-frequency flicker detection with the electronic shutter and also flash sync with the electronic shutter, at shutter speeds up to 1/180 sec (very close to the 1/200 sec possible with the mechanical shutter) and with either Canon Speedlites or third-party flashes.