An obstacle to wider adoption is the fact it's DragonFly-specific.
If you want to use ZFS you can pick your OS between Solaris, FreeBSD, NetBSD, as well as various Linux and Illumos distributions. Most people will be fine with one of them.
XFS, ext4, Btrfs are Linux-specific, but Linux is widely used, has first class hardware support, and comes in lots of different distributions of all kinds, one of which can probably suit your needs.
To use HAMMER you need to run DragonFly, which is not widely used at all and comes with significant drawbacks for production use:
- limited hardware support compared to more mainstream OS
- no binary updates (rebuilding from source is required)
- no long term support (only the latest release is supported)
- lacking documentation