I just read a paper by Axler and McGill (get it here) which compared 12 differend ab exercises their spinal compression force and the EMG intensity. Abmat situps where not analyzed. Is there any paper, which analyses the abmat situp regarding spinal compression (i.e. how safe it is for the spine compared to other ab exercises)?
Given that Dr. McGills complaints are around the spinal compression during a situp or crunch, given the similary of the movement I say a lot of the risks carry across to the Abmat situp.
For example, consider the below image taken from the AbMat Situp page:
You can see it encourages a forward flex of the abdominals to force the body up, and while the majority of flexion is in the thorasic spine, I'd suggest that by the nature of the movement there would be dangerous flexion in the lumbar spine as well.
If you are looking at ab exercises I'd recommmend following Dr. McGills advice around what constitutes safe and look at ab exercises that encourage strong static engagement of the abdominals.
Depending on your goals, deadlifts and squats will both encourage a strong core, similarly, for isolation ab rollouts, russian twists (where the spine is not flexed, but the body is rotated) or woodchoopers will all encourage sport-specific strength.