There is some cause for concern, it can be painful if it goes wrong. The worst case scenario is underestimating the drop height or improperly positioning the knot. If you don't break you neck and the knot is positioned at the back or sides of the neck then you risk being stuck with a painful suspension hanging. Another risk is not breaking the neck but rupturing the major blood vessels, which hurts a lot and causes death by internal bleeding in the head and neck. Personally I am overestimating the drop to ensure that doesn't happen. The only major downside to this is that it can cause decapitation, but I don't see that as an issue.
Overall, if you thoroughly snap the neck your brain will stop communicating with your heart and lungs. You should lose consciousness almost instantly and die within minutes with virtually no chance of being saved if you are left alone for a while and the damage is done properly.