-is it actually silent?
Yes, the electronic shutter-only mode of the GFX is quite silent. Nothing moves or clacks. Be aware, however, that a few GF lenses with internal image stabilizers might make some sounds. The GF 120mm comes to mind; it's positively alien.
-are there any strange artifacts or jello distortions when using this mode?
Possibly. "Rolling shutter" effects might occur if the camera or subjects move quickly during exposure. It's the nature of readout speed on the sensor.
-any other drawbacks? Does the image stabilizing still work in silent mode?
Yes, IS works during ES mode.
I highly recommend you experiment with the lenses you plan to use before committing. Rental is your friend.