it is a rolling shutter with all its limitations + using using rolling shutter in Panasonic cameras somewhat degrades sensor output (for example Panasonic switches to 10bit output from sensor)
The following Panasonic cameras have electronic shuttering. G5/G6/GX7/GH3/GH4/GM1
The first four in that list DO NOT switch to 10bit mode.
I have not read anything as yet about the GH4 doing it, and frankly, I'd be surprised if this would be the case for the top end model.
Its only the GM1 that seems to have some new strange implementation of sensor readout that drops to 10 bit.
as noted - using flash, hi quality 12bit output for stills, shooting moving objects, higher gains (specifically for JPEG shooters)
The problem that shutter shock gives you is that you lose SOME resolution when you're demanding the utmost quality from your images. If you switch to using flash, then for the VAST majority of people, the flash itself will freeze the scene, therefore you won't get shutter shock even when switching to conventional shutter mode. The more you introduce ambient light into the lighting balance, then yes, the more the 'shocked' sub image will START to become visible, but since we have a huge amount of people who claim they can't even see shutter shock in 'normally shocked' images, then fill in flash images will tend to have very, very low amounts of visible shock and thus reduced resolution.
As for the restricted use of higher ISOs - come on folks, get real - shutter shock's problem is that its robbing you of ultimate resolution. If you want to shoot these mirrorless cameras at ISO6400, then noise reduction is going to be smoothing detail so much, its really not going to matter if you have to switch to conventional shutter for these higher ISO values is it !?!
And frankly, the same type of argument goes for the rolling shutter effect. Sure, if you want to do stuff like pan and track fast moving cars, then yes, you might find the background buildings will get slanted. Well doh .... do you REALLY think that if you would have switched to conventional shutter mode for this and got a teeny weeny bit of shutter shock, that its really going to be visible in THIS TYPE OF SCENARIO ??
And there in lies the issue. Yes, a slow readout electronic shutter mode CAN be used in certain situations that can show the obvious visible effect of a rolling shutter or have other limitations, but in these situations, all you have to do is swap back to using conventional shutter mode, and in these 'certain situations' I bet that the vast, VAST majority of people would not be able to see the effect of any shutter shock even if they were to get it, because there are other factors involved that are degrading or limiting ultimate resolution anyway.