I am not sure. Some people mentioned that Btrfs NAS is better at protecting data. I read several people who bought DS220j or its former model ended up buying one with Btrfs. Not sure if it is a good idea to have it so I want to ask the experts here.
QNAP uses ext4 and OverlayFS technology, providing high-efficiency volume and block-based iSCSI LUN snapshots to well protect important data.
Challenges of using Btrfs for NAS applications
"Btrfs has greater I/O latency. In the same testing environment, file transfer via Samba with the ext4-using QNAP NAS is 61.5% faster than another brand’s NAS that uses Btrfs. In the benchmark report from Phoronix, a leading technology website for Linux hardware reviews, it reveals a similar test result. "