Originally ZFS could not be a boot disk because of the license issue. There was some other important feature that was denied to linux users, originally (forgot what it was). Apparently the booting restriction was eventually overcome. I don’t really grasp how the licensing changed that made booting possible.

Conceptually ZFS was relatively superior to all other filesystems. If it’s fully liberated, I don’t get why it is not more popular. I might expect it to be a default of sorts when installing Debian.