Honestly, you only have the ZFS port (which I don't know the state, stability and completeness, people said it hasn't evolved much since NetBSD 9).
WAPBL has proven not to be reliable/fully implemented (after reading tons of mailing lists about that)
FFS2 without WAPBL is not journaled so yes there is a risk and I already experienced this to test it with a USB key snatched off without unmounting: There is data loss.
Actually NetBSD does not even handle recovery when a volume is uncleanly removed (ripped of) without unmounting.
So removing completely the USB without unmounting literally caused the system to hang.
I made a post about that a few months back.
I know about these because I was trying to reproduce https://unixdigest.com/articles/battle-testing-php-fopen-sqlite-postgresql-and-mariadb-on-ffs-ufs-ext-xfs-and-zfs.html on NetBSD and used a USB drive for that purpose..
WAPBL was not useful at all on that side.
Honestly, just do recurrent incremental backups. (and/or ZFS/lvm snapshots)