This has the same risk as the newest version of SysUpdater. It just verifies that you're not screwing up yourself (verifies correct update pack, md5 checksums). As far as the downgrade process, that's not 100% safe in either version. No reports of hard brick AFAIK with safeSysUpdater tough.
If your downgrade pack is for the same 3DS model (o3DS = o3DS downgrade pack, N3DS = N3DS downgrade pack, not interchangable) then you're fine using that pack.
If you don't want to risk anything, then this is not for you.
Also note that if you get soft brick, you'll get 10.4 in recovery process. So no entry points except BrowserHax or NinjHax (don't quote me on that, not entirely sure).
It seems that people not on 10.3 have a tougher time downgrading sucessfully, most people get soft brick and then used to update to 10.3 and then were able to downgrade just fine. But that's out the window now that 10.4 is out