In addition to the alternatives already mentioned, I've been very happy with SnapRAID+MergerFS for a few years now. I don't have to worry about a magical black box as with btrfs or ZFS, I can expand the array with disks of any size, if one disk fails I only lose the data on that disk while the array remains usable, and it's dead simple to setup and maintain.

The only drawback, if I can call it that, is that syncs are done on-demand, so the data is technically unprotected between syncs. But for my use case this is acceptable, and I actually like the flexibility of being in control of when this is done. Automating that with a script would be trivial, in any case.