I too am a sceptic of Flying Saucers simply because they haven't been around long enough for us hedgie parents to observe any negative impacts (e.g. foot injuries) and I haven't seen any solid scientific proof to support it's safe for hedgehogs.
I've done a quick online search and found a number of medical and academic studies on the risks of running on a sloped or cambered surface in humans - many of the studies either reported or warned about injuries in knees, lower legs, and Achilles' tendons.
One can argue hedgehogs are not like humans, therefore, those studies are meaningless. I can't deny it and I'm not concluding the FS is neither safe or unsafe for hedgehogs precisely because of it.
What worries me is about a FS is this: (1) the studies I found describe the injuries caused by running on a cambered surface "slow and gradual"; the human subjects didn't even notice the pain until after having run on the cambered surface for several weeks. If the same thing is to happen in hedgehogs, would we be able to correctly diagnose and treat the injuries properly? In other words, injuries caused by the FS is not going to be obvious like a hedgie' toe caught in a mesh and ripping his toenail off, so how can we tell our hedgies are getting injured in their feet or legs?; (2) a FS is now readily available either in a pet store or online at a reasonable price. I understand they're quieter and easier to clean than a Comfort Wheel too. Because of these advantages, new hedgie owners may opt to buy a FS over a bucket wheel or CW. Without enough study done on the FS, it could hurt many hedgies before we discover it's not suited for hedgehogs after all.
Most of us know a bucket wheel is safe. So why not stick to it, rather than risking our hedgie's health with something we don't know very much about and seems to have good potential to cause injuries based upon the studies in humans?
I know chinchillas have been using the FS for years without any issues, but if you see video clips of them running on it, they run quite differently on it than a hedgehog does. For one thing, they hop/leap on it and their spine is not crooked or their feet are not crossing each other.
Those who have been using the FS for their hedgehogs say their hedgies just love the FS and it's completely safe. Personally, I think they're jumping the gun. I believe it takes more than a dozen hedgehogs and over a certain period of time to determine whether it's safe or not. I want a FS to be a good option for hedgehogs and their guardians alike, but I still feel the subject is in the "remains to be seen" category.
My apologies for the long post...