No hard brick since the new release of SysUpdater.
The problem with the first build is that it deletes all the CIA and it installs the part useful for the "Safe_firm" (recovery to upgrade to the last firm in case the downgrade fails) at the end. The problem is that if you've got a problem, you can't update to 10.3, you're screwed.
Now with the latest build, the safe_firm is installed first. Then sysupdater deletes a CIA and replace it, one after the other. (It doesn't delete everything and then install everything, it's CIA per CIA)
At the moment, except if the native_firm install fails (I don't think that it happened with the latest builds), I don't think you can hard-brick the 3DS. You can soft-brick, but you just have to boot into the recovery menu to update to 10.3 and retry.
It's not entirely safe but the risk is quite small atm.
I did a downgrade, 9.9E to 9.2E, no problem, not even at the reboot.
The most difficult part is to boot memchunkhax2 because usually it fails to get the permissions. Once sysupdater got those permissions, no problem.