Yes, I've been careful to have only one hub powered up at a time.
I'm not sure about chattiness, but they've been rock solid with my C7. I tried unplugging them, but only for 10 seconds or so
What 2.4GHz WiFi channels are you using in your house? What is the channel width setting for 2.4GHz WiFi? Sometimes Routers/Access Points will automatically change channels to the detriment of a Zigbee network. Again, making sure to only use 2.4Hz WiFi channels 1, 6, or 11 with a 20MHz channel width is the best way to avoid Zigbee interference on Zigbee channels 15, 20, and 25.
I just changed the Zigbee channel to 25 at a power of 8. I'll see how it goes.
I have six Unifi access points. They're set to channels 1 and 11. Channel width is set to VHT20.
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Ideally you want these farther away from your Zigbee channel - any reason you can't move the ones on 11 to 6?
The Linptech mmWave sensors you have aren't in the "chatty" groups, so unlikely to cause problems from that end. However, they do repeat, and I found on my C8 that they were bumping off some Aqara contact sensors, so I moved the Linptechs to my C7 and everyone has been happier in that setup.
I have three Iris v3 motion sensors on my C8-Pro (and more v2's), had them on my C8 before. So they can work on the C8/C8-Pro.
However...After I first got my C8 when it was released, I had to move a number of Iris motion v2/V3 sensors from my C8 to my C7. It wasn't that the device isn't compatible w/the C8 (as noted I have several in use on the C8/C-Pro) but something about how some of them were working w/the C8 just wasn't stable. After trying repeatedly to make them work I had to move on and put them back on my C7. No issues there.
Others have used v2 and v3 sensors w/out issues on their C8s from the get-go. I tried three different C8s when I was working through my Zigbee problems, similar on all three, so it appears to be an environmental issue to me. In the end I did find that I achieved stability w/Zigbee on my C8 when all of my Zigbee devices were connecting via repeaters, no direct connections.
The Unifi devices are set to nightly channel optimization, but I could turn that off and set channels manually. However, it's puzzling to have to blame my wifi when everything worked fine on the C7.
This is a disappointing workaround when a touted advantage of the C8 is external antennas that allow for more direct connections.
I'm waiting for my Zigbee channel change to 25 to kick in--it's been a couple of hours; no devices have connected yet. If this doesn't pan out, I'll do what you did and move Zigbee devices to the C7 in a hub mesh--might as well go with what I know works.
That is not a recommended approach to managing interference between Zigbee and 2.4GHz WiFi. I also use multiple UniFi APs. I have then forced to use channels 1, 6, and 11 with VHT20. I have zero Zigbee issues for my Hubitat channel 20 mesh and my Philips Hue channel 25 mesh. On my C8 Pro, I use power level of 8.
I believe that would be a good idea, to ensure things are not changing unexpectedly.
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I have now turned off channel optimization on the Unifi, with the access points set to channels 1 and 5.
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As @ogiewon indicates, not the best approach for use w/a hub, and IIRC not generally recommended in the Unifi community. C8 Zigbee 3.0 radio may be a bit more sensitive than C7, so providing some space should be a good thing.
I'd wait until tomorrow before doing anything else...
I only moved ones that weren't behaving well on the C8...I have >90 Zigbee devices on the C8/C8-Pro, including pretty much all types.
To make sure I understand how this works...If Zigbee devices are split between two hubs, they're two separate meshes, right? Meaning they can't act as repeaters for each other?
Correct, each hub mesh (Zigbee and Z-Wave) stand alone...
Well, 18 hours after changing the channel to 25, the Zigbee map was showing 10 of 19 devices. However, only one worked--a Philips Hue motion sensor, and that was intermittently unresponsive.
I ended up manually re-pairing each of the 19 devices. It's been a mixed bag. The Linptech presence sensors seem to be working. Two Aqara FP1 presence sensors paired fine but don't respond to motion. Several Philips Hue motion sensors work some of the time, but then stop reporting motion (stuck on either active or inactive)--although interestingly they seem to continue to report changes in illumination and temperature. Restoring full functionality requires re-pairing them again. One of the Iris v3 motion sensors reported motion for a time, then stopped. However, it does continue to report temperature
I'll give this more time for the Zigbee mesh to settle down, but this isn't promising.
My 2.4 GHz wifi is on channels 1 and 5 with a 20 Mhz width; my Zigbee channel is 25; and I'm in a rural area, 1000 feet from the closest neighbor. I'm not sure the interference explanation is the entire issue, particularly with the partial function I'm seeing--devices no longer reporting motion, but still reporting other things.
As much as I like these, they are in the extremely chatty category. Still testing them in my production.
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Probably a matter of "compared to" 
My context was other mmWave sensors that make the Linptech look calm, comparitively. I have four on my C7, had five until very recently, and didn't experience any issues w/them. But if you compared Linptech to a sensor like the Iris the Linptech are at a much higher level. Using @kkossev's drivers they have worked for me. 
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Chatty or not, I'm very happy with my five Linptech mmWave sensors. They've been solid, with both my C7 and C8 Pro, and they paired more easily than any device I've ever had.
Which brings me to a further update of my situation:
-After another day, the Philips Hue motion sensors have calmed down and seem stable.
-I re-paired one of my Aqara FP1 presence sensors, which is working. The other now won't pair (stuck on initialization)--I've tried at least 50 times, using every suggestion I've ever read about pairing, whether reasonable or probable superstition. [I tried booting up the C7 and pairing, which was immediately successful, so it's a C8 issue.]
-The Iris V3 motion sensors definitely don't work with my C8 Pro.
So, my plan is to replace the two Iris motion sensors and the one Aqara presence sensor. Hopefully the C8 Pro will be happy with that.
I don't recall having any significant problems with Zigbee devices before the C8 Pro. I was always wrestling with Z-wave issues, while Zigbee just worked. Not anymore.
It looks like I spoke too soon. An hour after I went around to confirm that all of my Zigbee motion sensors were working, all of my Linptech and Hue sensors (among other things) dropped off the network and stopped working. I now have only 4 of 18 devices showing as connected.
Time to take out the C7 and use it for Zigbee devices.
EDIT: removed inflammatory statement
I have all of my Zigbee devices back on the C7, which is sitting on top of the C8 Pro, and so far so good.
By the way, I looked at the C8 Pro's logs from this morning; the Zigbee radio looks quite unstable. Is this a hardware or software problem?
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That sure doesn't look normal but unfortunately not unheard of; one of those issues that have yet to be explained that predates the C-8 Pro.
Interesting that a couple of the online/offline intervals in the log appear to be 8 sec. in duration-- that aligns with the time it took to the C-8 to recognize Zigbee was down (due to a detected internal error) and reset it.
Discussed in this thread from a while ago: C8 Zigbee Radio Turning Off/On Multiple Times a Day - #114 by mike.maxwell
I'm clearly late to the party, since I didn't have a C8. I was only vaguely aware of the Zigbee issues with the C8, but now that I start reading the threads...oh my. (For instance, this)
What's frustrating is that, knowing there was a Zigbee problem with the C8, how was the C8 Pro released, presumably knowing it would have the same problem?
Of course I'm in no position to know... I don't recall seeing acknowledgement of any specific Zigbee problems regarding the C-8 since its release, aside from firmware changes said to address join issues in some scenarios.
C-8 seems to work well for lots of people; it's evident that a lot of effort was spent on testing migration scenarios which seem to always succeed. The Zigbee issues that keep popping up inolve new device joins which succeed then drop off a short while later, or devices that migrated fine but later won't rejoin after being reset.
They seem consistent with what would happen should a newly joined device fail to change its link key from the default 'well known TC link key' after joining when commanded by the Trust Center in the hub-- something older Zigbee stacks pre-R21 didn't do. R21 stack (which isn't new, dating back to 2015) and later are the 'Zigbee Pro' protocol versions which Zigbee 3.0 is based on.
As I understand it the Trust Center can determine if 'legacy' devices support TC link key updates and accommodate them if they do not... maybe something is getting missed in that communication,
The offline/online radio issue seems to be a side issue; IIRC some folks had their hubs replaced under warranty but I don't know the issue was resolved,.
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If it helps anyone in the same boat...I entered a warranty inquiry owing to my Zigbee problems and was contacted by bobbyD of Hubitat support. He felt my C8 Pro was being overloaded by an issue with the way it handled network devices, causing the Zigbee radio to shut down. (This fits with something else I've observed--repeated hub load warnings, even though CPU load % has shown as less than 20%. There are a couple of threads about possible false load warnings, but I wonder if the high loads are real, but transient.)
He had me install a beta firmware, which over the past 16 hours has kept the Zigbee radio from going offline. And so far, my Iris v3 motion sensors are working.
Thanks to bobbyD!
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