The wobbly element of the Swiss ball helps the abs to fire up properly. I find people are fairly useless at engaging their abs well, just gripping the abs, legs and shoulders like billy-o, assuming that such hard work must be correct – after all, no pain, no gain. When it comes to good ab work, which should cross over to activities, the story of how well working abs feel becomes very different. When suffering a running injury, like runners knee, people get the advice to strengthen their core. So the usual pile of sit ups, planks and bicycles follow, with abs a-bulging, breath holding in the plank and gritted teeth – and the pain in the knee will not get better, so eventually we give up running citing it not good for the knees.
So a well executed Pike up will be felt in the low abs and not be fantastically hard work – but fun and a good dynamic ab workout. It would be better to keep the head in line with the spine, so looking down at the floor in the the starting plank position, then, as the bottom is hoisted, looking at the Swiss ball. Much nicer for the neck too.