This crash might be related to disks' condition more then to UFS.At the moment I use SU.
Today a server crash and rebooted. Then it crash again few times (after running the fsck automatically at boot). I have seen the same issue in the past and I was able to solve it by booting in rescue mode (mfsbsd) and manually run fsck. Fsck took a long time as the gmirror was rebuilding.
I try to find if SU+J will solve this issue.
SU+j (soft updates journaling, to distinguish from the -J flag in tunefs, which is geom_journal) works pretty good.