Mine unsnapped on tower of terror, but I'll call that user error. I had my camera bag and another bag wrapped on the wrist with the band. We recovered it very quickly.
My husband's band came off on Space Mountain. We had to go to guest services to get a new one. No charge.
If I am carrying a backpack to the park, I throw a set of magic bands from a previous trip in the bag. I always make sure to have an extra set of magic bands from an old trip in the car in case one of us heads to the park without one (I learned this the hard way. This did happen once before a pre-park breakfast and it was a nightmare to get it replaced. Mostly because we went to the guest service window and were erroneously told to get in the breakfast line - -which I knew was wrong. When we got to the front of that line, they told us to go back to guest services.)
Even with these experiences, I wouldn't spend money on the tab thingies.