I only have 1.2 devices, I haven't yet reached 3.0 and I see that this will be the way forward.
I went down to 40 because that was the device number they have downstairs. What I did was set up another room on floor 1 and I have separate zigbee networks.
I share more information about my experience. Since I have 5 hub c8 I have the ability to experiment.
There are two c8 hubs that were defective, both of them apart from having a zigbee off radio problem, also after two days the hub remains completely frozen, it does not even respond to a soft reset, I had to remove the power to restart it.
Clearly, and with this I do not want to detract from the people of Hubitat who continue working and posting for the project, but in my opinion, equipment came with a hardware failure, they already sent me a replacement and I have to file a claim to have them change me the other.
I think we have to wait and see how they solve this problem, which does not happen to all C8s. I have another C8 running for several days without a problem.
For my part I will have to buy another C7 because these work like a rock.
Look at this case. This equipment is faulty, in total I have 78 devices 30 are with batteries the rest are Zigbee 1.2 router
And the routines I have are simple, you occupy 0.35% of CPU usage. I don't even have an RM set up.
Side question: what are you using to generate your mesh network map? The default one from Hubitat definitely doesn't look like that.
It's called zigbee-graph you can install it through Hubitat Package Manager
This post spawned HE to include this capability:
Still hanging in there...no Zigbee reboot since I booted up on June 14, so nearly three weeks.
Have had a couple of devices pause reporting briefly, but a Refresh on their Device page "woke them up" and they started reporting again.
So not perfect but worlds better than before, and my automations that rely on Zigbee devices are running w/out issue. I've also been able to join new Zigbee devices w/out issue, previously joining or re-joining Zigbee devices was an iffy proposition.
Ya every time i join or rejoin a device zigbee reboots. When i get home i am gonna have to request a new hub as well.
My improved results have continued, thankfully. Everything continued to work perfectly while my wife's family was visiting last week, to the point where she even bragged to them about a couple of my automations. (Be still, my beating heart.) 
OT: I also added a couple Z-Wave devices (iBlinds) and setup and automations worked w/out issues.
Looks like things are starting to settle, I myself haven’t had any issues at all with the radio restarts or the transition to the C8 overall, but still feeling pretty bad for all the people that have, nothing worse than constantly chasing issues.
Let’s hope thing continue to progress to the point of having a rock solid home automation system like the C7 and prior iterations.
My c7 seems to be relatively stable (moved almost two thirds of my zigbee devices back to that) but the c8 still seems to suffer regular reboots several times a day unfortunately. However with most of the crucial zigbee devices back on the c7, the reboots are now not of as great concern.
(Edit) But it does appear that the reboots are not as frequent. Only 6 over the past two days (rather than say, 6 in an hour which is more like what I used to have). Though this might have more to do with the fact that most of my zigbee devices have now been moved off it. Probably a good week or so before I finish moving all zigbee devices over to the c7.
I had the same experience. Started moving devices off. Reboots slowed. Eventually they stopped when I moved just about everything off. Doesn’t really solve the problem but now everything is running on the old C5 without issue.
So am I missing out on anything leaving it there?
The main benefit on the C8 should be the external antennas/better Zigbee/Z-Wave range, but since the C8 wasn't working well w/your Zigbee devices on it that's a moot point, obviously. If Z-Wave is working well on the C8 then I'd leave those devices on the C8. For me, anyway, Z-Wave on the C8 is better than on the C7. More direct connects, higher speeds, more reliable. If my Zigbee hadn't settled down, I would have moved all Zigbee to my C7 and kept my C8 for Z-Wave.
C8 is Zigbee 3.0, but again, if it wasn't working for you and the C5 does, then that Zigbee chip upgrade isn't relevant to your needs.
C8 also has Wi-Fi connectivity, but if you don't have to use that and it's best to use Ethernet.
I think C5 won't support Z-Wave Long Range, but you can do that on your C8.
Well, in my experience in these months with the C 8 is that the zigbee radio failure decreases as you remove devices, I have reached the number of 35 devices already stable in the C 8, we still don't know why that happens, now that the zigbee 3.0 version is better than the one that the C7 brings.
Regarding the C8's Wi-Fi connection, I highly recommend not using it, in my experience I have had a failure to the point that the C8 Hub hangs, with an ethernet cable it does not happen.
I know which team is working on it because I have had contact with it, I bought 5 C8 hubs and 3 were giving me a problem, in one they recognized that the hardware had a problem, and in another they uploaded an additional code to my hub to monitor some values.
Support sent me a new hub. So far it has been solid. If the shutoffs will start again, net step is downgrading to C7.
I had a similar experience to you at 35 devices . I added zigbee devices to my replacement c8 one at a time / 3 or so a day. Everything was stable with no zigbee reboots. It wasn’t until I hit just on 40 zigbee devices that the radio started having offline/offline events. For a good couple of weeks or so before that it was fine. I’ve since reduced the number of devices on the c8 as I move back to the c7 and have dropped well below 40 devices. But the reboots continued. Not suggesting that the number of devices have any bearing. Just why happened to me.
I've migrated all zigbee devices back to my C7 from the C8 due to the radio reboot issues. So far so good.
To those that received a replacement C8 hub, did the replacement hub suffer from the same reboot issues or did it fix it?
I'm just wondering if its possible that I recevied two "broken" hubs in a row as my replacement hub suffered from the same reboot issue. FYI the replacement hub I recieved may have been a used hub as it was unpackaged and missing the PSU and an attenane. Wondering if it may have been inadvertently returned due to the same issue.
Thanks.
[EDIT: clarified that the replacement hub "may" have been a used one. I only assumed that to be the case given it was missing parts, unpackaged etc.]
I don't think they send out a whole new "kit" in original packaging for warranties. Last I knew, they sent only the hub. Presumably your power supply and antenna and the ethernet cable from the old hub are good, so why send replacements.
Almost zero chance you got two bad hubs in a row. I doubt the first hub was bad either. The second acting just like the first would seem to support this fact.
You quite likely have a bad device, a device is somehow incompatible with the C8 chipset, or some integration or app on your hub is acting badly on the C8.
I had issues with certain cloud integrations killing my C8 when I first migrated. I had to turn off all cloud and LAN based integration for a while to narrow down what was causing the issue. It wasn't easy to track down, but once I found the rogue app, things calmed down and the C8 worked normally.
Download the Hubitat app
