I'm now up to 97 zigbee and 27 Z-wave devices on my C-8 with none dropping offline. On Zigbee I have:
7 Hue motion sensors
15 Iris V2 motion sensors
2 Samsung contact sensors (V4, V5)
5 Iris V2 contact sensors
4 Linkind contact sensors
6 Sonoff contact sensors
10 Lightify 4-button switches
9 Sylvania 2-button dimmers
4 Kwikset locks
4 Samsung buttons
2 Lutron Aurora rotary dimmers
2 Hue 4-button dimmers
4 GE in-wall dimmers
3 Sonoff Zigbee 3.0 dongles
1 Sengled Element Color Plus bulb
1 Tuya Valve (soon to be 3)
1 Tuya mmwave radar sensor
3 Samsung leak sensors
4 Linkind leak sensors
5 Samsung plugs (2-V4, 3-V5)
2 Hue plugs
3 Sylvania plugs
On Zwave I have:
4 First Alert zcombo smoke/co detectors (first gen)
2 Zooz Zen34 button devices
1 Ecolink leak detector
3 Ring gen 1 motion sensors
10 GE/Jasco z-wave plus dimmers and switches
1 Zooz Zen30 double switch (700 series)
7 Zooz Zen77 dimmers (700 series)
Things are working better than they ever have. I'm on Channel 19 with power level set to 20. No child devices on the hub's zigbee routing table. All zwave devices are directly connected and 100kb/s except for the First alerts at 40kb/s. I have a C-5 with 40 zigbee devices on channel 24, a second C-5 with radios disabled (that I migrated), a Hue bridge on 11 and another on 25. A couple of Velop MX5300 nodes on who knows what channel because Velop knows best (not user selectable), 3 HomePod minis and 3 ATVs (not currently using thread for anything, so I don't know if that's enabled as well). I have all of my sensors and lights connected to Apple Home via Homekit beta and Homebridge V2, Echo Speaks, Sonos, NOAA Weather Alerts, Google SDM API for my Nest Thermostat, Alexa, CoCoHue, MakerAPI, Home Assistant Device Bridge, Hubitat Package Manager, and of course the standard built-in apps for device control. This is a 2700 sq ft 2-story home built in the late 80s on a half-acre lot. Very little wifi interference from neighbors.
EDIT: I just scanned the wifi and my Velops are on 2 and 10, so maybe Velop does know best considering the different zigbee channels in use. That may also be why my one Hue bridge changed channels from 15 to 11.
I used a wifi analyzer app and found my wifi hubs are all on channel 2 (were -- now on channel 4). The neighbor seems to be on channel 8, with nothing else above that. My c8 hubs were on 18 and 20, with the one on 20 doing OK but the one on 18 dropping the Hue outdoor. So I moved 18->22, dropped the power to 8, and crossed my fingers. One Hue outdoor is still attached, but one dropped again. Then my c8 hub on 20 that was doing fine dropped both of the Hue outdoor attached to it. They had been working OK for the past few days. All my ST leak sensors are doing fine. I'm tempted to move up to channels 24 and 25, but I didn't have these issues with the C5 hubs (no idea what channels they were on), and it only seems to be the Hue outdoor sensors.
Don't know what to do at the moment, just hoping this information might help somehow. I'm sure the developers are working on this, I just hope they figure it out soon.
After a few days of running on the C8 my troubles with zigbee devices dropping off has abated. My only issues now are sensors that periodically report their battery state and temperature have stopped sending reports. z-wave devices were all working well from the very start of the migration but started to have slow or missed responses after a few days. I did a z-wave repair and a reboot and response time is much improved. It still isn't instantaneous like my C5 was but almost. Overall things seem to have smoothed out a bit.
This was the post that made me consider changing my radio channel to see if it would stop the C8 going offline every 2 min.
For 4 years I had channel 20 with no issues on my C5. But you have a lot of the same devices as I do and it they work on channel 19, hopefully they’ll work for me on that channel too.
So far the zigbee radio has stayed online more than 2 minutes 
Waddya think?
Unfortunately, not for me. Updated last night. This morning reset and rejoined my three misbehaving hue outdoor sensors. One is still reporting, two dropped after about 3 hours
My one hue outdoor motion is at, like, a day and a half.
I haven't gotten past two days yet.
Migrated today, super easy, all z-wave are Direct, all zigbee currently holding steady.
All my cloud integrations came back after re-enabling and re-auth. ES worked (I have a local Linux auth server) right off even refreshed the alexa cookie without fail.
I am continuing to see my devices show a status of inactive after having worked fine.
Background:
Formerly had a C5 and a C7. The C5 had only WiFi devices, the C7 had Zigbee. Hub mesh was used to share WiFi with the C7. All apps ran o ran on the C7. I had absolutely no issues with the devices, hubs, apps, or voice control (via Google Home integration). I migrated both to two C8 hubs. The C8 running WiFi devices have the Zigbee and Zwave radios disabled. Hub mesh is still used. All apps run on the C8 hub with the Zigbee devices.
Issues:
1. Devices go inactive:
- Samsung buttons
- Samsung muli-sensors
- Enbrighten (GE/Jasco) light switches and dimmers
- Innr RGBW Smart Bulbs
- Sengled Contact Switches (old and new)
I had the C8 hub (Zigbee) replaced under warranty. I am seeing the same issues with the new unit.
2. Sengled contact sensors - hard to pair, improper reporting:
I just tried to add 4 Sengled contact switches and am seeing odd and disappointing behavior. They all eventually paired after persistently retrying over 6 times on average. 2 show battery only. 1 shows the battery and the contact status but does not report any change in status when the window is closed or opened. It always shows open status. 1 shows inactive status after a few hours. I also added one to a door, it too became inactive after 2 hours of pairing it.
I am at my wit's end with Hubitat and the C8 now. After years of problem-free smart home operations, my wife is demanding it all be thrown out. I am desperate to get this all working. If anyone has an idea of how to resolve these issues, I would love to hear it.
