Been using BTRFS on a couple NAS servers for 4+ years. Also did raid1 BTRFS over two USB hard drives connected to a Pi4 (yes this should be absolutely illegal).
The USB raid1 had a couple checksum errors that were easily fixed via scrub last year and the other two servers have been running without any issues. I assume it’s been fine since they’re all connected to a UPS and since I run weekly scrubs.
I enjoyed CoW and snapshots so much that I’ve been using it on my main Arch install’s (I use Arch btw) root drive and storage drives (in BTRFS raid1) for the last 4 months without issue.