I like the idea. Would there be any expected performance differences between running ZFS on a Pi using FreeBSD instead of Ubuntu?
The great thing about zfs is the portability Linux to BSD and vice versa, feature flags noted. I've done it many times on racked servers.
The sad thing about the radxa card is that it presents as usb. You don't want to run any raid-like fs over usb if you can avoid it.
Beyond that, you still get all the other ZFS goodness for management, snapshots, compression, zvols, error detection, send/recv, and so forth. Best of all, you can always throw another disk at it and convert to a mirror if the situation changes.