[vetinari]

They do encryption/compression on subvolume level; each share you create is a separate subvolume.

For RAID5, they are using it on top of LVM, but with some modification - the synology implementation hooks LVM and btrfs together, so it gets ZFS-like properties.

There's a guy on the internet, who was playing around with it: https://daltondur.st/syno_btrfs_1/

[tobias3]

So they have fixed the last big hurdle to btrfs adoption in the small (single node) NAS space and are just sitting on it (violating the GPL). I urge any Synology user to write them to send you the Linux kernel source then upload it somewhere... though, their last Linux kernel drop seems to have been in 2017, so not much hope there...