Back in the day you might have been told to avoid situps because they can strain your lower back and neck. That could be true if you have bad form, but it is generally a myth to avoid doing situps in favor of crunches.
It is a myth perpetuated in the 1990s and early 2000s by foo foo personal trainers trying to convince middle-aged overweight Americans that they don’t have to do situps to get good abs, and in fact doing situps is ‘bad’.
Bollocks I say! We can file that one up there with high carb / low fat diets and curling in the squat rack.
To protect your neck, read the post on How to Reduce Neck Strain.
To protect your back, the answer is simple: keep your back as straight as possible throughout the movement, lay on a towel or mat to protect your tailbone, and avoid anchoring your feet as that can sometimes cause lower back strain.