Count me in on having the Zigbee offline/online issue, most gracious sir. 
Details posted most recently here:
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I have 36 of the xfinity sensors and they all work wonderfully
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Thanks for that. Also just noticed my NYCE motion sensor in the kitchen has also just dropped off. So another one to add to the list.
The 2 NYCE I have in my master bathroom have been solid
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Thanks. They were also solid under my C5. It’s only on my C8 that I’ve started to have issues.
There are issues apparently with the migration. If you started from scratch they would be fine. They're working on it. A lot of things were fixed I .125
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I have nearly a dozen of NYCE ceiling sensors, they have been by far the most solid sensors I have used over the years, including on the C-8.
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I agree. I have 5 and they’ve been flawless over the years on the C5.
I'd want support to chime in, but one option that you might want to consider is to reset the crap (sorry for using highly technical terms) out of your C8 and then redo the migration from your C7 using 2.3.5.125 on both hubs.
If you've mostly been fighting fires since your migration and haven't made a lot of changes, then the downtime could be a less than trying to fix what you've got.
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Devices dropping off may be the side effect of Zigbee going offline frequently. As @gopher.ny mentioned a few days ago, your specific case is still under investigation. We will follow up on your private message when we have an update. But if you don't hear back, don't hesitate to ping us, we get lots of PMs so it's easy to miss one or two.
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Thanks @bobbyD Much appreciated.
As a side question, if the issues are solely related to migration, would a complete factory reset and adding each device to the hub from scratch solve the problems? It’d be a big job as the c8 only holds the zigbee and Zwave devices and shares them to the other hubs which have the various rules and apps. This would mean I’d need to fix every single rule etc. on the other hubs that uses the devices so don’t want to use up a weekend to go down that path unless this would fix the problem.
Thanks again.
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I would hold off for a few more days until we complete the review of your case. Starting from scratch is a nuclear solution and I don't recommend it. Worst case scenario might be restoring the cloud backup again.
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Sorry for the delay in responding @danabw Must have my own mesh problems. 
and Mr Mayhem sounds very grand.
Up to 2.3.5.123 it has been a nightmare. Since latest furtle to 2.3.5.125 it has been good.
I have regularly been adding and removing Sonoff devices, as these cause most issues for me, between 4 hubs. 2 x c7 & 2 x c8. None of them have got stuck in “initialising” phase and pair quite quickly.
Also the Sonoff temp and humidity sensors used to stop reporting after a day or so and I had to un-pair and re-join to get it going again but that has also been solid since the release of 125.
I am biting the bullet today and rebuilding one of the c8’s as the primary node, this will be primarily z-wave but it will have a backbone of Sonoff sockets and push buttons ready for phase two.
Wish me luck
The issues have made me clean up my act though like removing my guest Wi-Fi (why did I ever need a guest Wi-Fi) to reduce unwanted 2.4ghz noise, spending time mapping the myriad of Wi-Fi trash from neighbours systems and selecting the best channels for the 4 zigbee radios, using common sense on positioning and really thinking about the environment around the kit has made a big difference on the stability and performance of the system.
e.g. lots of glass in the conservatory should be fine. Nope its Pilkington double glazed units that bounces back Wi-Fi like pee off a cling filmed toilet bowl.
So wish me luck for the big swap today. 
p.s. I will be clearing databases and resetting radios as I go as I found this stabilise everything. Only in my opinion of course.
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I migrated from a C-7 to the C-8 and have had zero issues.
56 zigbee devices, the bulk are Iris V2 motion and contact sensors, a few Iris V3 motion, a few Iris zigbee/zwave repeater plugs (disabled z-wave), a few ST leak sensors, an ST multi/tilt sensor, a single sengled bulb.
26 z-wave devices, primarily Zooz and a few Inovelli switches and outlets, GE/Jasco switches, a few dome sensors, and a couple go-control contact sensors incl. one that lives and (mostly) works inside my safe.
Small by comparison to some, but ES sure uses a lot of cycles so considering moving it to my currently shutdown and reset C-7.
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My free memory went up almost 100 MB after moving this to my spare hub.
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You can have your guest SSID broadcast on the same channel as your main SSIDs. There wouldn't be any difference in noise, provided you're not using the same frequency as ZigBee, etc. Just fyi.
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It's OK I don't have many guests except on visiting days
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Oh the memories of days gone by... Your guest wi-fi comment is funny for me, as I just re-enabled our guest wi-fi at my wife's request for her own family members that are visiting w/us for several days. She said "I don't want them to have our wi-fi password - they might accidentally post it on the internet!" I have no idea how they could/would accidentally do that, but it's her family, so...
I hope rebuilding won't be the path I have to take...not fun. Had my first significant C8 implosion late last night, both Zigbee and Z-Wave affected, just as wife's cousins arrived to stay w/us for several days. Got things up early this AM to a hub reboot seems to have mostly fixed things, and can't believe the timing...
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Enable the guest network as "Moochers" with the password of "7061726173697465"
