Risk with Situps
The risk is because of the repeated flexion in the lumbar spine, with very little results to show for. Many people also start to pull on the neck as they fatigue. Yes, the abs are engaged but we already spend so much time in flexion during sitting, lets not add more flexion on top of that. The risk is not large, but still why do it, if you have better alternatives?
Even the military has gotten away from using situps for strength testing.
Planking is a much better core exercise: we are working 360 degrees: abs, glutes, low back, obliques, shoulder and legs. Planking has taken over the situp. The problem is carry over to functional activities. Holding your core tight for 30 seconds or several minutes is not something you ever do in daily activities, sports or at work. Risk: low, some people have shoulder problems with these exercises. The reward is there, but its just not practical, nor functional
Functional, safe and effective. Period.
You can do a lot of variations and the core is challenged in an effective, functional manner. Upper body, lower body and everything in between is activated and you can use it as part of your cardio: your heart rate will go up as you start doing some reps to failure. Another way to progress your carriers is by walking on your toes. Bring in more calves and challenge your balance and posture