Matthew Dillon’s been working on “reliable on-media topology” for HAMMER2. If you had a crash at just the right time with HAMMER2, you wouldn’t lose data but you might have to do some manual cleanup. (Don’t ask me the steps; never happened to me.) With these changes, that doesn’t happen any more. It’s present now in -master and will be in what should be DragonFly 5.4.1 by the end of the year. He has a post to users@ that goes into better detail. If you want way too much detail, you can check the commits.
Three related notes: snapshots are now faster, the HAMMER2 design document has been updated to the tune of 400+ new lines, and yes, you can encrypt your root HAMMER2 filesystem, and have been able to for a while.