Adding native ZFS, allows for multiple pools, snapshots, vm enhancements (thin provisioning, backups without powerdown, linked vm;s), server 2 server backups (you can stream zfs), bitrot protection, zfs with l2arc and zil, De-duplication, realtime compression.... etc.
Unraid FS has its uses, multiple random sized discs, ideal for media storage etc, ZFS just ticks all the boxes for the other use cases.
Adding proper ZFS support ticks more than just 1 feature request
BTRFS is dead on arrival... anyone remember rieserfs ?
BTRFS is so experimental, almost every kernel release has had breaking changes and constant bugs.
BTRFS will kill itself in most situations and when you really want to tank all your data.... enable some "advanced" btrfs features, and power off your machine at the wall.
Ubuntu and Redhat use XFS as the default.
Ubuntu uses ZFS by default for containers.
Proxmox has been using ZFS for years.
Solaris and *BSD use ZFS
ZFS on linux, is similar to *BSD zfs, but vastly different.