My setup is also working fine … so far. I only had once the disk full, but this was because I forgot to mount the backup HDD before doing the rsync to it …
Nevertheless, I also use BTRFS on my notebook together with mint, and I noticed that there are some things that really can slow down the system or cause hangs (e.g. deleting snapshots with quote groups are turned on, the config that timeshift recommends).
Thats not the problem.
Raspberry pi and usb disks raid configured is asking for trouble.
In a raspberry pi with two usb disks, one for data and one for backups with a good self powered USB hub never had any problem running Nextcloudpi with those disks BTRFS formated in the last 3 or 4 years.
Is not possible to put the Eiffel Tower in a shoe box.
