I heard that the reason that emunand bricks on downgrading to 2.1 is that CFW just doesn't support it. So in theory, downgrading Sysnand to 2.1 is safe on an o3ds. N3ds never had 2.1 so it'd brick.
As of today luma3ds and reinand both support 2.1 downgrade on new and old 3ds.










