Hip Flexors don’t just lift up the leg. The Iliopsoas attaches to each of the Lumbar vertebrae and also functions to assist in maintaining the lumbar arch in its proper position.
Even McGill said that flexion is not necssarily a bad thing but that loaded flexion is no good. If you do situps by holding a weight at your chest (not behind the head) and contract your abs before you rise up the situp becomes a pure hip flexor exercise while the abs maintain a static contraction. McGill also said that by putting a pillow under the low back, bending the knees and performing a crunch with only the shoulder blades coming off the floor is safe.
As usual its not the exercise that is the problem, it’s the incorrect execution. If all we do is listen to the doctors and the PT’s then squats are bad for you, pulldowns behind the head are no good, deadlifts are dangerous, behind the neck presses will wreck your shoulders, bench presses will destroy the shoulders as will dips etc. The bigger picture here is to try the exercise correctly, then if it hurts you don’t do it.
Oh and don’t forget to tell Wendler and dave Tate that their both wrong in recommending hanging leg raises as the number one ab exercise for squatting heavy.