My experience is not indicative of most, but since you asked here ya go! Though on paper the C8 mesh looks much stronger, with most devices connecting directly, in reality I have a couple of lingering issues about 4 weeks after the initial migration.
(1) I have a couple of Ring gen 2 keypads (FLIRs) unable to maintain a stable connection whereas with the C7 they were fine for years. I actually tried pairing them to a second C8 and they exhibited the same issue, albeit with no repeaters.
(2) Some devices, most notably Zooz ZSE18s, showed really odd routes and slow response times.
(3) Some Aeotec trisensors started eating batteries like they were gummies.
(4) On the C7 every one of my devices had at least 2 neighbors. On the C8 I still have 5 that are only reporting 1 neighbor.
(5) Some devices are still slow to respond. I understand from Bryan that RTT for battery operated devices may not be accurate, but the ol 'walk into the room and note the delay before the lights come on" method indicates there are still some issues.
(6) I often get partial inclusions with new devices or a stick. The stick in particular will often show up in Simplicity Studio (now THERE's a misnomer) as included, not show up at all on HE, and see only a fraction of the devices HE reports. I can usually get it to work after a half-dozen exclusions and inclusions but with the C7 generally no issues.
(7) I've had some devices sitting on a desk literally broken into pieces and 24 hours later during a remove attempt HE thinks they are no longer on the failed nodes list.
I've taken a number of troubleshooting steps with mixed success. Bryan has tried to recreate the Ring issues and has not been able to. I suspect, though have no data to support this conclusion, that there are some devices in my mesh that just do not like the C8 and are causing problems they did not cause on the C7. Why, I am not sure.