My house is about 5700 sq ft or so. Using a single C-7 didn't quite provide coverage from middle of house to farthest extreme in the basement. So I added another and initially set up as mesh, with some devices paired to bottom floor hub, and rest paired to middle floor hub (top floor and middle floor were all paired to middle floor).
Adding Apple HomeKit integration (with the few others I have) caused the middle hub to report that it was overloaded (not sure the exact message but Triangle ! icon from same area that shows when upgrades are available )
So, turned off mesh, and disconnected all of the bottom floor zwave and zigbee devices from the mix and removed a number of devices that were shared into Homekit and was finally able to get it to be stable and not continue to show overloaded. Wasn't what I wanted but got things stable.
When I heard about the C8 and it having external antennas, thought maybe this would fix thing, at least from the signal distance (I do have more zigbee and Wave devices set in-between the Hubitat and the farthest devices, but not sure that they all can act as bridges). I did the migration and am now fighting just getting the C8 on-line with HomeKit (but I've posted that as another topic)
With 2 C7's and 1 C8, I'm trying to figure out what's the best way to utilize them. In the bottom floor I have some zigbee door sensors, a Rheem Wifi Hot Water heater, and a Z-wave home water shut off valve. I can move a hub into the utility room where the Hot water heater and water shutoff are, as that's where some house ethernet hubs are.
In the middle floor, a bunch more zigbee door/window sensors, 2 Schlage Z-wave door locks, a number of zwave in-wall switches (and a couple outlets) along with some plug in power plugs, etc.
Top floor is mostly zigbee door sensors along with at least one in wall z-wave outlet. I have a number of water sensors still to be re-connected into the mix (on all three floors) but haven't bothered with them til I get the hub arrangement sorted AND the Homekit issues sorted.
(BTW, I DID have all these devices connected and bridged into Homekit with SmartThings, but got fed up with the "which language are we going to settle on" that broke a number of integrations and caused developers to jump to Hubitat -- I'm happy with the Hubitat device, just haven't been able to get back to where I was with the SmartThings Hub)
I'm open, at this point, to starting over from scratch, or re-pair the other devices with the middle floor hub (with the external antennas) -- the smart things hub I had before I went to Hubitat was able to reach from end-to-end in the house (and top to bottom) so I know it's physically possible with zwave/zigbee.
The hubs have hard-wired ethernet (it's easily accessible on all three floors). Not planning on using Wifi for the C8. I bought it mostly because of the antennas.
Should I just focus on the C8? Will C7's be able to act as "extenders" of some sort without mesh? I didn't have a great experience with Mesh turned on with HomeKit.... I'm trying to get back to where I was with the SmartThings hub and thought with 3 hubs I should be able to extend and handle more devices....