TRUTH:
Crunches and it’s variations are effective for building strength and hypertrophy of the rectus abdominus muscles (6 pack muscles) however the ‘abdominal muscles’ have a complex function and must be trained in different ways.
The ‘abdominal muscle’ or ‘core muscles’ group which is made up of several different muscles have all the following functions: breath function (most important function) stabilize the spine and pelvis, dissipate forces to and from the upper body, they work in flexion, extension, lateral flexion and rotation of the spine, maintain position of internal organs etc.
Train your abdominals first in all 3 planes of motion (frontal, saggital, transverse) for example: side plank, front plank, paloof hold. Once you’ve established a foundation with these exercise add in crunch and oblique crunch variations depending on your goals. Strength training regularly also aids in strengthening the abdominals!
Do not use endless abdominal exercises in a futile attempt at reducing the size or shape of your stomach the single best way to see visible difference in your abdominals is with a reduction in body fat.