Synology uses it in production. At least for some features
Using it in production on hundreds of machines for several years now. Zero issues so far.
i use it personally, but found reiserfs works better for me on spinning disks - both space and speed. ( lots of small files ) - btrfs, even with compression uses more space
for normal workloads, ext4 is fine for me
People using Qubes OS find btrfs fast and reliable: https://forum.qubes-os.org/t/btrfs-and-qubes-os/6967, https://forum.qubes-os.org/t/ext4-vs-btrfs-performance-on-qu...