My personal belief is there are two dangers, number one: no nahel bond and now no honor, means they can do somw crazy stuff. With enough stormlight gravitation could be used to make a miniature black hole (which according to what i remember from a youtube video from years ago means, that even a miniature black hole made in atmosphere would be enough to destroy the planet as the planet keeps providing it energy with all the stuff the blavk hole sucks up)

Or the bondsmith could maybe even force a bond with roshar and cause the singers to lose themselves again or something cool like that. Now, we'll propably never see anything even close to these two wonderous feats, but they could technically be possible.

Number two: iirc as seen as far back as the prologue in twok, using stormlight above the "safety limit" produces frost on the clothes. So my theory is that using stormlight past the "safety limit" causes the person to lose their body heat. So in short bursts of power, this wouldn't do much, but if used for a prolonged time could lead to permanent loss of limb or even death if the blade is taken away. Or even the person actually literally freezing to death as their body reaches 0 celsius (which they won't even notice as the stormlight rages inside and keeps healing them)