where did you see/ read that ? on these stupid commercials ? when some stupid people try to sell their stupid products ?, well the truth these old techniques are the best techniques , you will hear all kind of crap from people for some reasons , i train my abs for many years i never had any problems with my back

except some few injuries from a t-bar , presses behind the neck , chin ups with 60 pounds and up attached to my belt , very heavy rowing with bar , etc , so the weight and body temperature played a role , i guess i used to lift heavy without warming up , i think doing the crunches it`s the best was to develop a great abs (upper parts) and leg raises for the lower parts , and side crunches for your obliques , i used the machines they just my waist/abs bigger so i don't recommend that , same thing goes for the roman chair with weights .
take a look at this link to perform a great crunches
http://www.bigmusclesfast.com/exercise-crunches.html
i hope this helps