But lacking raid5/6, even N-way mirrors. ZFS is not perfect, but btrfs is not even close in terms of features.

Btrfs does support raid5/6, I'm using it right now. It is still being refined and has a couple rough edges, but I haven't had any problems in the year or so I've been using it. It is not "production ready" yet for sure, but the support is there.

Everything I've read (status link from the official wiki: http://marc.merlins.org/perso/btrfs/post_2014-03-23_Btrfs-Ra...) says don't touch raid5/6 yet. Maybe the pages are overly pessimistic but lack of recovery features sounds like a no-go for me?

in a enterprise setting you rarely want to remove devices/space.

Rarely is not never.