@TomAoki I have not used OpenBSD. My note was a regurgitation of the general response to the question of interchangeably using FreeBSD UFS on OpenBSD and/or vice versa.
I will change my tune if (general) you would have happened to use latest stable version of FreeBSD UFS[2] on OpenBSD (or, OpenBSD file system on FreeBSD)
for 2-3 years
on a regular basis
instead of native file system
without issues that are not related to such use.