I currently have a C5. I'm thinking about getting a C7 and migrating my 700 series devices to it and leaving the 500 series devices on the C5. I'm not really having issues with my hub, but some of the zwave improvements would be nice QoL updates. Namely being able to see the route a device is using an the neighbors graph, as well as the ability to run a zwave repair on a single node instead of the whole network.
But I'm not really sure how having two hubs works. Will this fragment my zwave network into a C5 and a C7 network? I don't really want that. Or will everything still be part of the same network? Is this what hub mesh is supposed to do?
A lot of the issues I see people post about has to do with trying to include a device with no security and Hubitat not allowing that so they need a separate zstick and have to include the device using a PC. Is this mainly for older 500 series devices that also supported S2? If I split it up so that only 700 series devices are on the C7, will I have these type of issues?
Hub Mesh is just a LAN integration between hubs (if you've used HubConnect or Hub Link/Link to Hub, it's similar in end result, just a different way to get there in that it's built-in and not app-and-driver-based). So, you'll have two separate Z-Wave networks, as well as two separate Zigbee networks if you also have that on both hubs.
The C-7 allows you to include (most) devices with no security; the issue is a small set of devices that support security but only S0 and not S2 and don't have a separate method for secure vs. non-secure pairing. On the hub side, the C-7 does not allow you to choose no security for these devices, even though S0 is often recommended against (except for devices where you need security, like door locks or garage doors, and this is the only kind you can get). Notable finds include the Zooz 4-in-1 and the Inovelli bulbs, though a firmware update should have helped with the Inovelli ones. Zooz, I think, is just going to come out with new hardware (the Q Sensor seems to be a stopgap for the moment but also addresses this issue).
In these cases, using a secondary controller that supports this, like Z-Wave PC Controller with a USB Z-Wave stick, will let you get around the S0 issue, at least. But it's not a universal issue for all or probably even most secure devices, and 500 vs. 700 per se doesn't matter side from the fact that 700 requires S2 for certification, so you shouldn't have this problem with any of those devices. Many 500 series devices support S2 and lots support S0 but have a different pairing sequence for that, so they would also not be a problem.
Regarding the "problem devices" I mentioned above, some people still report bad luck with these particucular devices, even without security, on a C-7. It should be noted that Hubitat's note about the lack of compatibility with the C-7 doesn't indicate anything about security, and I'm not sure I've ever seen staff confirm that this was the only problem (though it certainly can't help). I believe the Zooz Double Plug, which also has issues if using S2 on the C-7, is one that is known to behave if you pair it without and disable power reporting (maybe that last part is just my preference, but it can't hurt if you don't need it )--but I'm not sure about the rest.
I have 5 hubs (Mix of C-7 & C-5) all meshed together. I have one main hub that runs my rules, handles my Alexa interface, Lutron interface, IP2IR interface, Homebridge, Lock code manager and contains my groups/scenes, etc. It consolidates all of the devices from the other hubs. It has local Z-Wave and Zigbee devices as well. The "remote" hubs all tie into it via hub mesh and it is solid. I have one C-5 hub that does nothing but Zigbee RGBW devices, they are happier that way. With Hub mesh you wouldn't know that the devices aren't attached directly to the main hub. The only downside is that once a "remote" hub goes down for reboot, etc it take a little bit before the devices that are meshed to come back online on the main hub. The C-7 is definitely where you want your Z-Wave devices. The tools are great! With that said, I wouldn't hesitate to use hub mesh between C-7's and C-5's interchangeably. Just to clarify each hub supports its own Z-Wave and Zigbee networks and they do not tie in with the other hubs networks. Hub Mesh does the work of replicating the devices behind the scene. In fact on a couple of my hubs I have Z-Wave turned off completely and use them for Zigbee only.
I mentioned this in another thread but I have 3 hubs online currently - 2 C-5's and a C-7. I use the C-7 for all Z-Wave Plus devices. Since the C-5 has the same Zigbee radio as the C-7 I kept my Zigbee devices on that hub. My other C-5 is my network hub - kind of like @automation's setup. I use this for network and cloud devices/services. Note: I had the 3rd hub on hand so decided to incorporate it.. depending upon your use-case you would not really need to do this.
On the other hand I have one of my clients running with a C-7 controlling all (ZW, ZB & Cloud) devices and a C-5 in the detached garage controlling devices around that location. Did this to alleviate range issues with some devices.
Both types of setups work great.
On slightly different topic I would definitely consider getting a UZB-7, an inexpensive but powerful tool to have in your arsenal...