No risk is involved throwing a tracking knife to the head a safe distance. Titans have to play strategically to land that thing, and it only usually lands on people who can't use their eyes or ears and the event if a huge -blam!-ing monster charging your ass. Handheld supernova requires you to charge and really isn't a big deal as it's easy to dodge and doesn't have an exotic to keep recharging it. If you miss SC, you die. If you miss Supernova, you die. If you miss a throwing knife, it's okay sweety you'll get it next time.