The biggest qualm of the Lumix G line is the autofocus. While serviceable for most still work, the AF system has had flaws that are most noticeable in wildlife/fast-moving genres and video. The biggest question with the GH6 is, is it improved? The answer is yes, but by how much, well, probably not enough to erase concerns.
Panasonic utilities a Depth-from-Defocus (DFD) system that uses depth and micro changes in focus to analyze a scene. This differs from many AF leading systems on the market that use a phase detection system, and many hoped Panasonic would go this route.
It turns out, the new DFD system in the GH6 is actually quite great for still work. It is vastly improved for wildlife photography, but users will still find phase detect AF systems, like that of the Canon R5, to be much better. But, outside of fast-moving subjects, the AF system in the GH6 works great.
The DFD system relies on a lot of information and frames for analysis, and consequently, video users will be disappointed that the AF system isn’t much of an improvement in 24p modes. However, the system works much better at higher frame rates (more frames, more data). Whether or not this is a big issue for video remains to be seen, as many videographers use manual focus regardless of the AF system.
The GH6 is a great stills camera, a nice upgrade over the GH5 and G9. The new sensor has better noise handling, dynamic range, and image quality than its predecessors. However, Panasonic also has done away with some still-centric features and modes, like focus stacking. There is a Handheld HiRes mode, which allows one to take multiple exposures handheld and the GH6 takes those, merges them, and outputs a file up to 100MP. Pretty cool.
The GH6, like the GH5 is undoubtedly a video powerhouse, thus I think we will actually see another new camera from Panasonic in the near future that is geared more towards stills. Much like how the G9 came a few months after the GH5 (the G9 had great video features, but was more stills focused).