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.