I can't speak to whether they're dangerous or not, because I don't ever use them. I see absolutely no purpose for having one on a folding knife, and I make my own sheaths so I don't need help deploying a fixed blade either.
And regarding retention of the blade during use, perhaps it's different for someone in active combat or something but for us normal folks, if you're unable to grip your knife you should either get yourself a better knife, or rest until you're able to get a better grip? Seems like either situation (slippery knife or tired user) is a cause for concern in and of itself.
Your right about cause for concern. When you experience degenerative arthritis, you might be more open minded about using a fob. [emoji41]