According to System Information, my refurb 2018 Mini has the Broadcom 4364B0 Bluetooth chip. Have never used a Bluetooth keyboard/mouse with a Mac before, but decided to give it a try with this new Mini, to keep more physical ports free. And since there have been a few threads about bluetooth issues, I figured that Apple couldn't claim any future problems were the fault of my 3rd party peripherals.
I have some mixed feelings about this. First, when I start up the Mini it literally never accepts my password on the first try, evidently some characters aren't sent when the keyboard first wakes up. Is this normal? Have gotten in the habit of typing a few random characters, then clearing them first. This works and is only a minor annoyance. The Magic Mouse is OK, but I've tried these in the past and didn't especially like them. At times, I have noticed that it isn't entirely responsive. This is generally when something intensive is happening, like copying files from an external SSD. Perhaps this is the interference issue that others have mentioned?
I'm running Windows 10 in Parallels and the Magic Mouse works fine in Windows Explorer, but it's a disaster with the specialized GIS software that I use. The lack of a middle button is a major problem. Tried a couple third party apps that provide middle-click capability and wasn't happy. Too hard to find the "right place" to click, I only had about a 50% chanceof getting it right. And the lack of tactile scrollwheel feedback was an even bigger problem. I really need to zoom one wheel-click at a time, so I can wait for the software to catch up rendering a complex map. Just can't do that with a touch surface, I end up over-shooting.
Out of frustration, I plugged in my POS $15 Targus USB 3-button mouse and it works like a charm in Windows. So, I'm using the USB mouse more often now. At the times when the Magic Mouse "stutters" (as mentioned above), I find the USB mouse still works fine. I'm just not sure whether it is "interference" or if there is additional processing for a Bluetooth device that slows down when there's lots of I/O happening. Anyway, my Apple Space Grey keyboard and mouse look great and they work, but not sure that they were the best choice - not sure that Bluetooth was the right choice either.