Make sure you are not using your hands to pull your head or neck up. This may lead to neck problems and pain.
Use a steady motion that does not cause your body to rock as you exercise to make sure that the focus stays on your abs and that the crunches do not become a meaningless motion carried out by momentum.
This exercise puts stress on your lower back. it is advised not to do it if you suffer from lower back problems.
The air bike much like the crunches involves spinal flexion which can cause a lot of stress to your spine and lower back.
But even worse than that, it also includes rotation. The worst move you can do for your cervical spine is a repeated flexion and rotation combined. That's the exact mechanism that causes a herniated disc in your lower back.