Both BTRFS and XFS write support is supported by 'sysutils/fusefs-lkl' on FreeBSD. No need to port anything.

ZFS is superior to BTRFS - first of all its safe and secure - I do not know how many more years wil ltake BTRFS to finally properly fix their RAID5/RAID6 implementations to not loose data .... and that also gives warning for general usage in any other way.

In which way iptables/nftables is better when IPFW or PF?

The only feature that KVM has against BHYVE is live migration ...