I finally got my BlackMagic (BM) ATEM mini and here are my findings.
First thing to remember DO NOT put an external mic into any of the mic input ports on the back of the ATEM mini and expect synchronous sound.
This was my first mistake, fortunately I was running all 3 GH5s with SD cards so this was not an issue.
You must set the main camera as your sound source in my case with the Panasonic XLR unit on the GH5.
There are 2 ways to preview your incoming shots the 1st is to set up your ATEM mini via the software via a laptop the second is via a second monitor with a loop through HDMI socket. There is no multi preview on the ATEM mini as it stands.
The 1st way relies on the USB-C output (Webcam out) via a computer so in my case not a practical way of doing things.
I have two BlackMagic Video Assist 4K monitors, using one as a preview and one as a recorder. The preview monitor only allows you to preview one camera but thats better than nothing.
I am not recording anything LIVE and all the 3 cameras are setup and static. A live recording would present a lot more difficulties in terms of preview. It has been suggested that BM may develop a multi camera output for the ATEM mini via software that would be a big improvement but as long as the lighting stays the same and you are not filming a theatre production the ATEM mini is a great tool.
It upscales all HDMI inputs to your chosen output via the BM software, in my case 1080 50p.
Although I have not used it yet it seemingly has a terrific chroma key facility once again driven via the BM software.
To work the software you need the following ports. A spare USB socket and an ethernet socket. Dont worry if your laptop does not have an ethernet socket as you can buy an ethernet to USB adapter online for about £15.
The only cable that comes with the ATEM mini is the 12v power adapter.
You have to supply your own USB to C adapter cable and the ethernet cable if you want to control the ATEM mini from your laptop.
The software also gives you access to the audio with a lot more control than you get with the ATEM mini itself. The controller is well made and does not come across as a cheap £300 mixer. To me this is a scoop at the price for anyone producing any form of HD programming.