I recently "upgraded" from C7 to C8 and have had nothing but problems. I have multiple zigbee devices that work for an hour and then become unresponsive. power cycling them sometimes helps, but these were solid before the migration. I also have the Alexa error messages constantly, but I'm more concerned with zigbee devices that seem to work just find and and then become unresponsive.
Also wondering what problems I'd run into if I went back to C7 which was significantly more stable than C8
This is kinda like stating the obvious but the C8 should work the same or better. I'm not an expert here because my Zigbee have been rock solid but I suggest you post the Zigbee mesh map (new capability in C8) Also check you have the latest firmware (even if you C8 is brand new)
If you left the C-7 powered down and didn't do any of the resets, then powering off the C-8 and powering on the C-7 would generally work. The same caveats for migrating to the C-8 apply going back... some clicks to re-establish communications (Alexa, etc.) might be needed.
I'm with @JohnRob though. My migration from a C-5 to a C-8 was more or less flawless and has been so since. I had two C-5's running different parts of my home, and then migrated to two C-8's. One had only Zwave, the other has both Zigbee and ZWave. Even the horrible zigbee Hampton Bay Fan Controllers (HBFC) have been perfect on the C-8.. they weren't on the C-5. I believe the better, external, antennas help the HBFC the most.
The migration appeared to be flawless until automations started to fail because devices became unresponsive. whats strange is they become responsive again if I power cycle them several times, or try to factory reset them, but it's getting old really fast. Network was working really well until the C8 came along.
go to logs then click on hub events. Anything interesting in there? (Do this first)
For initial diagnostics I would disable ALL lan devices (alexa etc) and integrations and drivers. This is key. Then under yourhubip>>settings>>zigbee details click rebuild network.
The reason is some lan integrations can cause an overload of the hub and the 1st thing the hub does is shutdown zigbee to recover.
I have had the opposite experience. For me, the difference between the C-5, C-7 and C-8 have been close to nothing... from the standpoint of having a house full of automations work for 3 years. Sure the new C-7 and C-8 offer better ZWave utility by having device by device repair and other platform improvements. But looking at my Kitchen Lights as ONE example, that automation hasn't been affected in the slightest despite 3 different hub models. I can say the same for the other hundred plus devices too. The new antennas have helped me, by allowing more devices to connect directly. And eliminating one hop MUST have benefitted the Automations, but to me as a person walking around the home expecting automations to work... I see no outstanding difference.
As someone who hangs around here hoping to be helpful, it's frustrating to hear stories that are so far removed from my experience. Worse, I never even experienced it to offer advice. 
I got the C-8 and tacked my most complex hub of four to migrate. It has 40-50 ZWave devices and 20-25 Zigbee. I couldn't migrate via Cloud, so I had to do a local backup and restore on the C-8. Then, after powering down the C-5, I went through all the Zigbee devices, since those are easy, once you know what the 'dance' is to get a specific device reset, then through ZWave, rebuilding the mesh close to the hub outwards, AC powered before Battery. Again, the hard part was finding the manuals online for my specific vintage of device. For the most part, manufacturers have stuck with a method, so a GE outlet and a GE Switch and a GE Dimmer all need the same clickies. There were 3 devices where that wasn't true. But at the end of that day, everything was identical to what it had been the day before. I never got to experience any "all my devices are dead".
+1
My opinion, based upon my experience with the C8, is that some LAN devices aren't migrating nicely for some reason. Some apps/devices may need a reauthorization, and others may need to be uninstalled and reinstalled to work correctly.
There seems to be a silent failure where these LAN devices/apps are trying to communicate, and they cannot for some reason, and they overwhelm the hub by trying to do so.
I would also consider moving some of these very chatty devices/apps to your old hub. That seems to be the optimal solution, these occasionally troublesome devices are isolated to their own hub so the main hub can do the important stuff.
This may be one reason I haven't seen any of this issue.. all my Internet Facing devices are on my 4th Hub where the Z-Radios are disabled (not OFF 
) Perhaps I started from a better "lane".
interesting, sorry you are frustrated that others have problems with migration or the platform overall. What a strange response.
I only have 70 or so devices on my network, and most of the are working just fine. I never stated that "all my devices are dead". For me around 7-10 are frequently dropping into an unresponsive condition. Since I haven't added anything new except the C8, logic would suggest the C8 is reacting differently than its predecessor. I was considering just removing all devices and starting over with the C8, but with limited time to do so I bought into the migration is easy story and now paying the price.
thanks for the useful reply, I don't have much in the LAN side, one Sonos device and Alexa - but I'll give that a shot. Is the C8 more susceptible to lan integration overload?
It should be easy and painless, but for a few users, and for some unknown reason, migration isn't always smooth.
I don't mean to put words into csteele's mouth here, but I think he is sympathetic to your issues, and wishes he knew how to resolve them. It is tough to watch people struggle when they shouldn't be. I don't think that was a strange response at all.
The C8 is identical to the C7 except the Zigbee chipset and the external antenna. If you read his and my response above, it appears to be some issue related to migration of these LAN devices. Your other thread with the Alexa issue very well could be causing all these problems with Zigbee.
I would start by going through every LAN device/app, especially Alexa since you already noted problems with it, and be sure it is working correctly. You may have to simply reauthorize things (click "Done" in the app) or you may have to remove and add these apps again.
Very good words.. thanks. That's just what I meant.
No but depending the the drivers, it may be looking for something and not seeing it. For instance in my case it was my iotawatt. I'd noticed that my zigbee was going up and down a lot. My iotawatt had gone off line (turned out to be a bad gcfi) and the driver was retrying it a lot with no indication in the logs. It would only log after the final time out. This obviously overloaded something ion the hub and the zigbee radio shut down temporarily to recover. Just saying start with disabling all lan integrations and drivers and see how things go.
Did you ever get this Alexa error message fixed?
Did you try to reauthorize the Sonos integration?
I didnt have any drivers to remove, but I've removed Sonos, HomeKit, Alexa, Hue. Random zigbee devices still become unresponsive, two or three each day need to be power cycled in order to reconnect to the hub. Zwave devices seem to be much more stable, I don't think they've had any problems. C8 has really screwed up my zigbee devices, with about 90 of them this is a real problem. Feels like C8 migration is the problem, as I didn't have these issues before, I had problems a year ago with zigbee repeaters causing issues and removed all of the "bad" repeaters (not a cheap fix) and now after "upgrading" to C8 i'm wishing I had not.
Honestly I have 60+ zigbee devices and I've migrated twice. (From c7 to c8 and c8 to c8 and have yet to have any issues with any of my zigbee devices save 1 and that is getting sent off to @mike.maxwell to look at (water sensor won't stay connected). I suspect you have a rouge device flooding the mesh. Do you have any power reporting going on?
no resolution found so far, same issues with devices dropping or being unresponsive - I've started to rethink the Habitat platform. Regarding the Alexa error, I've not found any solution to that, although my issues remain with or without the Alexa integration. seems like whatever changed from C7 to C8 has broken that integration. For a home thats mostly operated by voice commands this is a big problem, but it appears hundreds of Habitat customers have the same problem and the solutions presented are not effective.
whats the best way to determine that a rouge device is flooding the mesh and why would this only be a problem on C8 and not C7?
I think "rogue" in this sense is a device that happens to have it's parameters set for say power reporting, that are too permissive. It's not a rogue device so much as a rogue flood of reporting. Again where rogue means unexpected more than illicit.
For example, someone might say, "oh, I don't have anything that uses Relative Humidity%, so none of my devices report that"... only to find via logs and event counts, that there is at least one device sending every second or two. ( I picked RH% in this example to demonstrate it could be any number of parameters. ) Why would it affect the C-8 more than the C-7... well it shouldn't BUT if the rogue device connection to a C-7 is two hops, and on the C-8 it's direct, then the better antenna is the cause/difference. I'm not saying this is the issue, just that there is a way to cobble together a 'story' that weaves through the clues. 
Download the Hubitat app
