Caveat: I'm only at F1.
The issue is not lack of progressions with the ab wheel. There are many progressions with the ab wheel. Ab wheel on your feet (not knees) with one-arm (not two arms) and downhill (not flat or uphill) wearing a heavy weighted vest with a hold in the extended position anyone? :-) Ross Enamait on Rosstraining demonstrates many progressions with the ab wheel.
Therefore, the advantage of the front lever over the ab wheel is something else than progressions of the exercise itself. Ross is a boxing trainer whereas Coach is a gymnastic trainer. Therefore, it seems reasonable to assume that the advantage of the front lever over the ab wheel is progressions with other GST exercises. I cannot explain it better. I hope someone else will.
Lately, I try to focus on GST and only afterward play with other strength exercises such as the the ab wheel. I'm still only at F1 but I make faster progress than before by focusing on GST and I enjoy that. If I just could improve my flexibility (range of motion) as fast ...