The RAID 5/6 implement itself is quite stable. The only problems I know of with it are the scrubs are still ridiculously slow and free space reporting still doesn’t work.
That said I couldn’t use RAID 6 on mine as the main array because of the speed problem. My RAID 10 scrubs at around 1TiB an hour the RAID 6 around 100GiB and hour.
BTRFS on a whole though still doesn’t have disk monitoring or a way to drop flaky disks.