When I was testing I had the band fail for FP+ at Pirates of the Carribean. The CM kept telling me my daughter could ride, but I couldn't because my "FP" had already "been used". It took a bit to get sorted out.
The band wouldn't work on my room door (at first), nor would it work to get me to the atrium level I was staying at. I had to use the card for that.
The family in front of me had issues with charges and FP+ as well and significant issues making ADRs. In fact, when we talked about the website, I made a joke about "seeing Donald a lot", which they immediately got. Donald is the failure screen when the system dies on you...which happened a LOT when trying to make Dining Reservations.
The father's band couldn't charge, but the children's could.
They were not impressed. Overall, I think the system is a decent enough idea. I don't mind the bands or MDE. However, considering the capital that went into it and knowing what that could have gotten in terms of increasing ride CAPACITY...I can't say it was solid.
It made very little difference on my trip, outside of the few hassles. While anecdotal, these are my personal experiences with the band system failing. I've heard and read a lot more...