I have a mixed view of my a7sII.
First, it can keep a central subject smooth on something like a fast 1200 hp boat running through a wake.http://russellrutherford.com/fast_skif.mov
The central subject is level, the rest warps or jellos, though in all fairness this was shot at about 60 knots in very active conditions, though just slightly smooth walking can jello the file and panning must be kept very slow.
The motion file is low bit rate, and will band in the highlights. Now the standard thought would be to slightly underexpose and people rave about its low light, high iso capabilities, but mine doesn't.
In fact I did a test in London last week giving it one more chance and did some test stills at 1200 iso at night and compared it to an Olympus em1 mark II and the em1 had more detail. The sony caked and layered the subject like pushing a file to 8000 iso.
The battery draw on the Sony is crazy. It will burn through 70% of a battery just setting the menu up.
Grading the Sony is tough . . . takes 7 to 10 nodes in resolve to get a decent file and is not that great on skin tones. The only upsides of the sony is its build quality is much better than the first version.
Actually the best lens for it especially if you use any autofocus is the Zeiss/Sony 24-70 A mount with the pdaf adapter. The issue with using the adapter is you have a very small focus area and in video set to manual exposure you can't lock down the controls. It goes to some type of auto exposure mode and moves the shutter speed where it wants and also locks the f stop to 3.5. Very strange.
The Olympus up till the new one was not meant for video through the battery works longer, the menu of the oly is a mess, but it doesn't jello as easily and the stabilization is very good.
Personally though I haven't tested it I'd go with the new Panasonic G5 when it's available. The new panasonic has a 5 axis stabilization and up to 400 mb/s 422 10 bit, vs the Sony's tops of 100 mb/s 420, 8 bit.
Once again I have not tried the gh5, but own two gh3's tested the gh4's and the file is more robust and the camera is very easy to set up. Its like a mini 5d2.
If there is only a downside of micro 43 it's getting to super wide lenses. There is a leica autofocus 12.5 (25mm equivalent) and if you want to be able to have softer backgrounds you usually need to go to the manual focus voiightlanders at f .95.
The nice thing about micro 4/3 with a light cage is it's easy to mount them in cars and small areas and the gh5 has full sensor readout to 4k and uhd, rather than just crop the center of the sensor.