"stable" != "mature"
In development since 2007, stable since 2014. ZFS, in development since 2001 (correction I erroneously listed 2005 earlier), stable since 2006 (or at least Solaris included since then). Do you consider the Solaris years or just the Linux years and then do you consider the Linux Debian/Ubuntu sanctioned years or the ZoL years?
I'm fine with mature since included in the default installer on Ubuntu/Redhat/Oracle/SUSE/etc... for my definition of btrfs maturity.
As you can tell I'm not that strong on ZFS other than what I've learned in some small tinkering and discussion with others.