> For whom? For me data consistency is much more important
That's fine. I'd like data consistency too. But I still don't mount my disks with -o sync, nor does pretty much anyone else, even most of the people who say they want data consistency. That's the reality that fs developers live in.
Maybe on SSD (where nilfs2 is designed to live), we'll be able to get guaranteed data consistency as a matter of course. That'll be nice if it happens.