World Perks is the only program that you can book your Skyteam reward ticket on-line. You can check and book flights availablity of all Skyteam airlines yourself without calling FFP help desk of the airlines.
In Flying Blue, for instance, you can only book Air france and KLM flights online. And the chance of getting a free seat on a long-haul flight is slim, if you choose mini-miles option.
OnePass is not much different form FB, and SkyMiles is as bad as FB.
Based on my experience, award success rate is also higher at WorldPerks.
As an example, my girl friend who WAS a member of now defunct FD program, could not get a KLM flight on her FD online rewad booking tool last year. When I searched for the same flight for my girl friend on my WorldPerks site, it showed KLM flight as available. This was not even rule buster miles.
Others may think differently. So many people, so many minds.