Synology also has a bit of a software moat with its BTRFS-backed SHR implementation. You can throw in drives of arbitrary size and it'll automatically maximize the available free space. ZFS can't do that, though AnyRaid should make it possible in the future: https://docs.hexos.com/blog/2025-05-22.html
Plus, DSM has a spectacular web interface.
SHR is not exactly rocket science, it is the combination of lvm and mdadm. You can do that yourself on ubuntu.
Synology adds some additional logic to recover data from other RAID drives when the BTRFS layer encounters a checksum failure. I don’t think there’s any way to homebrew that part.