I was not acquainted with "EFCS" (Electronic Front-Curtain Shutter) so I did a search and this is the first article I read which also contains a video...
https://petapixel.com/2018/12/07/psa-electronic-front-curtain-shutter-may-be-quietly-hurting-your-bokeh/
Here is a direct link to the video...
https://www.youtube.com/watch?v=LQTarMu_y6Y
This is totally new to me. It appears that the use of an EFCS adversely affects one's bokeh. As I understand the article and video this issue appears at about 1/1,000th second or higher shutter speed when using an EFCS and seems to be a feature of all camera brands.
I wonder what others think of this article and video?
Dave
In the Z cameras (other than the Z9 which only has full electronic shutter) you have the option of Auto EFCS which chooses between EFCS and mechanical shutter operation automatically depending on lens, shutter speed etc. In the auto mode it probably avoids EFCS at high shutter speeds avoiding these bokeh issues. In some Z lenses there is no explicit mechanical option (e.g., 105 MC), only auto and EFCS.
Nikon do not in most cameras allow EFCS at faster speeds than 1/2000s (not sure about how things work on the 105 MC when at high speeds, if it does go to mechanical or continues with EFCS); this should avoid most of the bokeh issues.
I've personally not encountered any bokeh issues that I would have noticed but with PC lenses at wide apertures and with tilt in use, there can be some image shading (gradient) in EFCS mode.
EFCS should be the default operation mode on most mirrorless cameras as otherwise the camera would complete two cycles of the mechanical shutter for every shot, increasing vibration. EFCS also reduces shutter lag (in mirrorless cameras and in some DSLRs in live view).
The Z9 of course makes EFCS irrelevant for its users. For those cameras which don't have fast electronic shutters, EFCS can dramatially enhance sharpness, to the point that one can struggle to view through the somewhat smeared mechanical shutter shots.