I've played with most of them, I'm primarily a nixos user. I haven't yet had an experience as smooth as the FreeBSD installer for ZFS. looking at the debian wiki it still looks like dkms and manual partitioning are in order.
Same as anything else installed as a binary package - you trust the people packaging/providing the binary. If you don't, build it yourself. The source is publicly available.
Sure, but ZFS is much better integrated into FreeBSD. It supports ZFS on root with boot environments out of the box.
And when running a Samba server, it's helpful that FreeBSD supports NFSv4 ACLs when sitting between ZFS and SMB clients; on Linux, Samba has to hack around the lack of NFSv4 ACL support by stashing them in xattrs.
You can arguably get even better ZFS and SMB integration with an Illumos distribution, but for me FreeBSD hits the sweet spot between being nice to use and having the programs I need in its package library.
But on Linux you need to load external modules. Before upgrading or changing kernels you need to check if ZFS supports it. Specially bad in rolling distros.