A balanced workout would include a modest number of spinal flexion exercises and, then, only if you have a healthy back, as well as few rotation exercises to target the obliques. (cathe.com)
It's important for you to limit flexion in this pose by resting your chest and belly on your thigh, because excessive spinal flexion in combination with a sidebend can be even more dangerous for the disks and other soft tissue around your spine than excessive flexion on its own. (yogajournal.com)
Crunches, sit ups and other spinal flexion exercises are fine but as your spine moves in a variety of directions, you need to select a variety of exercises if you want to maximise your abdominal development. (fitnessandpower.com)