I'm only asking because my N3ds for my friend ... I've got it because I should downgrade it...As of now, the worst you can do is hard-brick due to user error, or soft-brick due to the app/unknown factors. Provided you aren't an idiot, it seems the app itself is safe (from hard-bricks). I still haven't managed to downgrade without soft-bricking the device though.
By the way, the o3DS from my Brother (FW 9.6.0) , I've downgraded it at the first try and it runs RxTools Emunand 10.3.0 Now ^^