Before I push the update button is the new C8 working with .121. I am not willing to go down another rabbit hole, I have reservations on making things worse that it has been.
I have had things stabilize after i moved the devices that were dropping to the C7 and used it as a mesh hub.
I am having a Chromecast issue with .121 possibly and is being looked at by @gopher.ny
I have backed out to .118 and that apprears not to cause the problem.
YMMV
This is a very broad question what kind of problems are you worried about. The latest firmware should force your zigbee statck to the right one now incase it somehow got messed up..
It SEEMS better on the zigbee side - still issues on the z-wave side, at least with me.
I had issues with the Hue stuff, and other zigbee, no issues with Z wave, I just don't want to go down another time wasting rabbit hole, if it is working better I will update.
My hue sensors have been fine on 121 since I installed it yesterday. They were also holding up very well and 118.
But so much of this stuff is a YMMV situation that I think all you can do is take it back up of your current setup and then if you want to, give 121 a try.
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If release notes do not include a fix for an issue affecting the hub, there is no pressing need to update. New version reminder in the top right corner can be dismissed, too.
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My rule of thumb is to wait until sequential releases are at least 7 days apart. For me, seeing updates pushed out every 3-4 days indicates that the bugs are still being worked out. But I'm still running a C7 and am not having any issues that need fixing.
I'm on C8 latest FW. Cannot pair zigbee devices. Wasted loads of time on reboots, channel changes, removed all devices, downgrades...nothing working. Currently feeling like a massive waste of almost £200!
They'll get there, it will be fine in the end 
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Conversely, if it isn't fine, then it isn't the end.
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I just got my C-8. My C-7 is on .121. I have all Z-Wave + devices, mostly Inovelli switches with a couple of Aeotec range extenders, a couple of Aeotec 7 smart plugs, and some other brand smart plugs, and on the Zigbee side I have 2 Hue bulbs that go through a Hue hub and 1 Hue motion sensor that connects directly to my Hubitat.
I do not have the subscription backup and will be using the migration tool to install the “special” backup. When I do switch should I update the C-8 before applying the backup to the C-8 or vice-versa, and apply the backup and then upgrade the C-8?
Also, I purposely waited a few weeks to get the C-8 due to the expected inevitable bugs. And know I want to know, in the words of Sir Lawrence Olivier from Marathon Man, “Is it safe?”
If you can not pair anything and your devices are on the compatibility list, I would suggest you make sure support is aware. You should also make sure your hub is using the correct Zigbee Stack. That is one improvement in the latest firmware. It will now force the C8 to the new stack while the C7 and below will be forced to the standard stack.
There is also a known issue for folks that use some devices that are not officially supported with known issues like Aqura. Simply put the better radio and antenna in the C8 makes those devices compatibility issues worse as they can more easy talk directly to the hub and bypass measures used to improve reliability.
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I'm running .121 on both my hubs. C7 and C8, went from .118 to .121 and no issues to speak of.
I don't believe there have been anymore zigbee changes between the two versions.
Yes, update both hubs before doing the backup and restoring the backup. There have been various tweaks to the migration in some of the updates as people have uncovered small issues.
Working for me.
It's believed that those experiencing signficant issues are a relatively small portion of users, but (and it's a big but) if you are one of them you will not be happy. Those issues can include both Zigbee and Z-Wave. If you are hit by them, you won't be happy, partner & household members will not be happy, dog might even be upset. 
So as long as your eyes are wide open on that...
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Cats, on the other hand, can see in the dark - so they're good.
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L-O-L
Plus they hate humans, and so delight in our frustration and folly. 
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