We don't have enough data to really know what the success rate is of flip-floppers, and they are very hard to track.
Up until 2015, the vast majority of flip-floppers were those who were originally north bounders starting in March and April. For some reason or another, they found themselves running out of time, and flipped either because they had to, or didn't want the time pressure of racing the clock to get to Katahdin in time.
Last year we had an intern who worked at ATC in the fall. She started a northbound thru-hike, but flipped from Harpers Ferry because she was getting bored in Virginia and wanted a change of scenery and some drama back in her hike. It worked! Her hike was successful because she split it into two separate pieces.
That's not to say that some might not lose momentum by splitting up their hikes--I've certainly heard of that happening.
To me the hardest part of my northbound thru-hike psychologically was hiking through the mid-Atlantic in summer. The heat and the humidity were oppressive (so much so that it was hard to even sleep), the views were hazy and washed out, the trail seemed developed and just not all that special. I asked myself why I was even bothering hiking, and what was so special about thru-hiking anyway. (Since then, I've hiked the mid-Atlantic in spring and fall, and it can be quite lovely).
Last night I was talking with a friend who's hiked the A.T. almost 5 times and has spent thousands of hours on ATC trail crews. He said if he did another thru-hike, it would be a flip-flop variation that would avoid the heat and humidity of the mid-Atlantic in summer and minimize exposure to Lyme disease.