The best results are obtained by maintaining a full range of motion during exercises, e.g. full sit-ups.
Crunches are for strippers, to get a six-pack you need to reduce body fat.
Mens Health is full of tat, myths, and half-facts gathered from the internet.
If you have sufficient strength to use free-weights with a full range of motion and correct form this will strengthen your "core" muscles, or fixers and stabilizers as they should correctly be called.
One of the best exercises to do is the Back Squat. Performed correctly it uses over 250 muscles. As it uses so many muscles it is a good fat burner. Performed incorrectly, it lands you in A&E.
Heath and Fitness is an industry selling products, just like any other industry. Most of it is marketing and pseudo-science.