All exercise has the potential for being counter productive. Using proper progressions and technique are absolutlely necessary. Many instructors teach entrie movments without progressions. Mechanical evaluation is also important on all exercises.
As to the curl up, all such ab exercises have gotten a bad rap because of one study done on pig spines that were put through thousands of full range flexion/extension. The study is so flawed. But it got attention in the media and is now part of “fitness urban legend”. If you understand biomechanics, anatomy, and physiology at all; you should be able to evaluate any exercise for any client.
I actually have heard of this concern, but I believe that it began from a source who didn’t truly understand the movement. It was being compared to the straight-legged sit-up, which we all should have the controversy and studies related to this exercise. The concern was the supposed strain that the rollup could pace on the cervical spine and back, which is the concern with straight-legged sit-up. The only true contraindications that exist with the roll-up are with persons who are pregnant, have osteoporosis or spinal stenosis. Otherwise, I believe that like all exercises, if done properly, there should be no concern with this exercise.
I agree with Joanne and my clients with healthy backs roll up and down. The movement is introduced through baby steps. It’s not what you teach but how you teach it that makes the difference.